cy 101coordination compounds1
DESCRIPTION
qweTRANSCRIPT
Coordination CompoundsCoordination Compounds
Why atom combines to form chemical bonds?Why atom combines to form chemical bonds?
Octet Rule: Pauli Exclusion Principle, Hund’s rule and Aufbau PrincipleOctet Rule: Pauli Exclusion Principle, Hund’s rule and Aufbau Principle
Reason: minimum energy and maximum stability (Reason: minimum energy and maximum stability (s- and p-normal saltss- and p-normal salts).).
Is it so important to study Coordination Compounds?Is it so important to study Coordination Compounds?
ImportanceImportance: : Many coordination compounds are found in biological systems. e.g. Many coordination compounds are found in biological systems. e.g.
Haemoglobin (Fe-porphyrin, red), Chlorophyll (Mg- porphyrin, green), Vitamins B12 Haemoglobin (Fe-porphyrin, red), Chlorophyll (Mg- porphyrin, green), Vitamins B12
(Co-complex), Cytochrome and Oxydase enzymes (Fe-Cu-complex). (Co-complex), Cytochrome and Oxydase enzymes (Fe-Cu-complex).
Adrenaline, citric acid and cortisone complex with metals (e.g. Adrenaline, citric acid and cortisone complex with metals (e.g. Pb, Cu, Fe, CrPb, Cu, Fe, Cr), which gave ), which gave
metal poisoningmetal poisoning and EDTA-M complexes used in and EDTA-M complexes used in treating metal poisoningtreating metal poisoning..
[Cu(NH3)4]2+[Cu(NH3)4]2+ ionion inhibits the growth of fungi and bacteria. inhibits the growth of fungi and bacteria. [RhI2(CO)2]- ion[RhI2(CO)2]- ion is used as a is used as a
catalyst in the "catalyst in the "Monsanto ProcessMonsanto Process" for making acetic acid, the active ingredient in " for making acetic acid, the active ingredient in
vinegar.vinegar.
Na-EDTA compexesNa-EDTA compexes: Soap, beer , mayonnaise. : Soap, beer , mayonnaise.
EDTA4- EDTA4- is used to "trap" trace amounts of transition metals that could potentially catalyze is used to "trap" trace amounts of transition metals that could potentially catalyze
the decomposition of the product. the decomposition of the product.
Colors on a computer Screen, DVD, CD, camera, electronic goods, etc. Colors on a computer Screen, DVD, CD, camera, electronic goods, etc.
Molecular or Addition CompoundMolecular or Addition Compound:: Stoichiometric amounts of two or more Stoichiometric amounts of two or more
stable compounds join together. E.g. Fe(CN)2.4KCN (Pot. ferrocyanide), stable compounds join together. E.g. Fe(CN)2.4KCN (Pot. ferrocyanide),
KCl.MgCl2.6H2O(carnallite) and FeSO4.(NH4)2SO4.6H2O(Mohr’s salt), KCl.MgCl2.6H2O(carnallite) and FeSO4.(NH4)2SO4.6H2O(Mohr’s salt),
etc. etc.
Types of AddTypes of Addnn. Compounds. Compounds::
1.1. Double salts or Lattice compounds:Double salts or Lattice compounds:
2.2. Coordination or Complex compoundsCoordination or Complex compounds: : composed of a metal atom or ion composed of a metal atom or ion
and one or more ligands (atoms, ions, or molecules) that are formally and one or more ligands (atoms, ions, or molecules) that are formally
donating electrons to the metal center. donating electrons to the metal center. Explanation is complex, because Explanation is complex, because
each coordination compound has slightly variable chemical, structural each coordination compound has slightly variable chemical, structural
and physical behavior depending on metal ion and ligands. and physical behavior depending on metal ion and ligands.
Coordinate covalent bond/Dative bondCoordinate covalent bond/Dative bond: : A Lewis acid-base reaction in A Lewis acid-base reaction in
which neutral molecules or anions bond to a central metal atom (or ion). which neutral molecules or anions bond to a central metal atom (or ion).
Ligands (complexing agents):Ligands (complexing agents): Lewis bases - contain at least one pair of Lewis bases - contain at least one pair of
electrons to donate to a metal atom or ion. electrons to donate to a metal atom or ion.
Within a ligand, the atom that is directly bonded to the metal atom/ion is called Within a ligand, the atom that is directly bonded to the metal atom/ion is called
the the donor atomdonor atom. .
Ligands may be +ve (NOLigands may be +ve (NO++), -ve (X), -ve (X--) or neutral (NH) or neutral (NH33).In the case of mixed ligands, ).In the case of mixed ligands,
complex ions gave isomeric structure and geometric shapes. Bidentate ligands complex ions gave isomeric structure and geometric shapes. Bidentate ligands
gave optically active isomers. gave optically active isomers.
Monodentate Ligands:Monodentate Ligands: Lewis bases that donate a single pair of es to a Lewis bases that donate a single pair of es to a
matal atom. It may be anions (usually) or neutral molecules.matal atom. It may be anions (usually) or neutral molecules.
Bidentate ligandsBidentate ligands: : Lewis bases that donate two pairs ("bi") of electrons Lewis bases that donate two pairs ("bi") of electrons
to a metal atom. to a metal atom.
Bidentate ligands are often referred to as Bidentate ligands are often referred to as chelating ligandschelating ligands ("chelate" is ("chelate" is
derived from the Greek word for "claw") because they can "grab" a derived from the Greek word for "claw") because they can "grab" a
metal atom in two places. A complex that contains a chelating ligand metal atom in two places. A complex that contains a chelating ligand
is called a is called a chelatechelate. .
AmbidentateAmbidentate: : more than one donor atoms in the same molecule.more than one donor atoms in the same molecule.
Polydentate LigandsPolydentate Ligands: : Having more than two donating sites.Having more than two donating sites.
Some Monodentate LigandsSome Monodentate Ligands
ligandligand namename ligandligand namename
FF-- fluoride fluoride ionion ClCl-- chloride ionchloride ion
BrBr-- bromide bromide ionion II-- iodide ioniodide ion
HH22OO waterwater NHNH33 ammoniaammonia
OHOH-- hydroxide hydroxide ionion COCO carbon carbon
monoxidemonoxide
CNCN-- cyanide cyanide ionion SCNSCN-- thiocyanate thiocyanate
ionion
Complex ionComplex ion: If the coordination complex carries a net charge, the complex If the coordination complex carries a net charge, the complex
is called a complex ion. is called a complex ion.
Coordination compounds and complex ions are distinct chemical Coordination compounds and complex ions are distinct chemical
species - their properties and behavior are different from the metal atom species - their properties and behavior are different from the metal atom
or ion and ligands from which they are composed.or ion and ligands from which they are composed.
Coordination sphereCoordination sphere:: a coordination compound or complex consists of the a coordination compound or complex consists of the
central metal atom or ion plus its attached ligands. The coordination central metal atom or ion plus its attached ligands. The coordination
sphere is usually enclosed in brackets when written in a formula. sphere is usually enclosed in brackets when written in a formula.
Coordination numberCoordination number:: The number of ligands bonded to the metal or M-ion The number of ligands bonded to the metal or M-ion
is called Coordination Number (CN) of the metal and the bonding calledis called Coordination Number (CN) of the metal and the bonding called
Coordination BondCoordination Bond (CB).
Methods of Studying ComplexesMethods of Studying Complexes
1. 1. Electrical conductivityElectrical conductivity: depends on concentrations and no. of charges on : depends on concentrations and no. of charges on
complex.complex.
2. 2. Cryoscopic measurementCryoscopic measurement: freezing point changes of a liquid.: freezing point changes of a liquid.
3. 3. Magnetic moment/propertiesMagnetic moment/properties: gave no. of unpaired es.: gave no. of unpaired es.
4. 4. Dipole momentDipole moment: structural information for non-ionic complexes.: structural information for non-ionic complexes.
5. 5. Electronic Spectra (UV-Vis):Electronic Spectra (UV-Vis): for energy of orbitals and shape of complex. for energy of orbitals and shape of complex.
6. 6. X-ray studyX-ray study::
Structure of Coordination CompoundsStructure of Coordination Compounds
The arrangement of ligands determine their structure, physical and The arrangement of ligands determine their structure, physical and
chemical propertieschemical properties
e.g. [CoCle.g. [CoCl44]]2- 2- structure might be:structure might be:
a. Sq. planar-ligands present at the corner of a squarea. Sq. planar-ligands present at the corner of a square
b. Tb. Thh-ligands present at the corner of T-ligands present at the corner of Th h
c. Something else? Experimentally found Tc. Something else? Experimentally found Th h
1. 1. Werner’s Coordination Theory (1893)Werner’s Coordination Theory (1893)
Alfred WernerAlfred Werner (1866-1919) (1866-1919)
1893, age 26: coordination theory1893, age 26: coordination theory
Nobel prize for Chemistry, 1913Nobel prize for Chemistry, 1913
Addition of 6 mol NHAddition of 6 mol NH33 to CoCl to CoCl33(aq)(aq)
Conductivity studiesConductivity studies
Precipitation with AgNOPrecipitation with AgNO33
Compound Moles of ions Moles of AgCl(s)
“CoCl3.6NH3”
“CoCl3.5NH3”
“CoCl3.4NH3”
“CoCl3.3NH3”
4 3
3
2
0
2
1
0
Co
NH3
NH3
NH3
Cl
NH3 NH3 NH3 Cl
Cl
Cl– attached to NH3 may be dissociated
Proposed six ammonia molecules to covalently bond to CoProposed six ammonia molecules to covalently bond to Co3+3+
Compound Moles of ions Moles of AgCl(s)
[Co(NH3)6]Cl3
[Co(NH3)5Cl]Cl2
[Co(NH3)4Cl2]Cl
[Co(NH3)3Cl3]
4 3
3
2
0
2
1
0
NH3
Co
NH3
H3N NH3
NH3H3N
3+
3Cl–
H
N
HH
M
ligand
N forms a coordinate covalent bond to the metal
(coordination sphere)
(counterion)
Coordination compounds structure:Coordination compounds structure:
(i)(i) 11o o ValencyValency: Ionizable bonds, (ii) : Ionizable bonds, (ii) 22oo Valency Valency: Non-Ionizable bonds e.g.: Non-Ionizable bonds e.g.
[Co(NH[Co(NH33))66]Cl]Cl3 3
Generally, coordination number (CN) varies from 1-12. However, 2, 4 Generally, coordination number (CN) varies from 1-12. However, 2, 4
and 6 are the most common. and 6 are the most common.
CN=2 linear structureCN=2 linear structure
CN=4 Sq. planar or Th structure CN=4 Sq. planar or Th structure
CN=6 Octahedral structureCN=6 Octahedral structure
Why Transition metals form Coordination Complexes?Why Transition metals form Coordination Complexes?
Sidgwick (EAN rule)-Sidgwick (EAN rule)- Because TM has vacant d-orbitals, which can Because TM has vacant d-orbitals, which can
accommodate electron pairs to gain stability like next noble gas config. accommodate electron pairs to gain stability like next noble gas config.
e.g.,K4[Fe(CN)6], EAN=36(Kr); e.g.,K4[Fe(CN)6], EAN=36(Kr);
[Cu(CN)4]-3, EAN=36(Kr); [Cu(CN)4]-3, EAN=36(Kr);
[Ni(CO)4], EAN=36(Kr);[Ni(CO)4], EAN=36(Kr);
[PtCl6]-2, EAN=86(Rn).[PtCl6]-2, EAN=86(Rn).
ExceptionsExceptions: [Fe(CN)6]-3, EAN=35; [Cr(NH3)6]+3, EAN=33: [Fe(CN)6]-3, EAN=35; [Cr(NH3)6]+3, EAN=33
Role of d-orbitals in the Complex formation: Role of d-orbitals in the Complex formation:
(i)(i) d-orbitals shape and degeneracy: d-orbitals shape and degeneracy: In case of In case of isolatedisolated gaseousgaseous and and free free
metal ionmetal ion all the five d-orbitals are degenerate. all the five d-orbitals are degenerate. These orbitals are oriented These orbitals are oriented
in space as shown below:in space as shown below:
Shape of d-OrbitalsShape of d-Orbitals
Shape of d-OrbitalsShape of d-Orbitals
Bonding in TM ComplexesBonding in TM Complexes
Theories for Metal to Ligand bonding in complexes: Theories for Metal to Ligand bonding in complexes:
1.1. Valence bond Theory (L. Pauling, 1930)Valence bond Theory (L. Pauling, 1930)
A complex involves reaction between Lewis bases (Ls) and a Lewis acid (M or A complex involves reaction between Lewis bases (Ls) and a Lewis acid (M or
M-ion) through coordinate covalent or dative bond. M-ion) through coordinate covalent or dative bond.
Assumptions:Assumptions:
1. Central metal atom or ion have a number of empty s, p, and d orbitals. On 1. Central metal atom or ion have a number of empty s, p, and d orbitals. On
hybridization, gave hybrid orbitals. hybridization, gave hybrid orbitals. These are vacant, equivalent in These are vacant, equivalent in
energy and have definite geometryenergy and have definite geometry..
2. The ligands have at least one 2. The ligands have at least one σσ-orbital containing a lone pair of -orbital containing a lone pair of
electrons.electrons.
3. Hybrid orbitals of the metal atom or ion overlap with the filled 3. Hybrid orbitals of the metal atom or ion overlap with the filled σσ-orbitals -orbitals
of the ligands to form ligand→metal of the ligands to form ligand→metal σσ-bond. -bond.
This coordinate bond is a special type of covalent bond shows the This coordinate bond is a special type of covalent bond shows the
characteristics of both the overlapping orbitals and Polar in nature due to characteristics of both the overlapping orbitals and Polar in nature due to
donation.donation. Pauling Pauling measured magnetic moment to find out the number of measured magnetic moment to find out the number of
unpaired electrons in a complex and the geometries of the complex ions unpaired electrons in a complex and the geometries of the complex ions
having the central metal ion with configurations d1 to d9.having the central metal ion with configurations d1 to d9.
Metal or metal ionMetal or metal ion: Lewis acid: Lewis acid
LigandLigand: Lewis base: Lewis base
Hybridization of Hybridization of ss, , pp, , dd orbitals results: orbitals results:
C.N.C.N. GeometryGeometry
44 tetrahedraltetrahedral
55
66
44
HybridsHybrids
spsp33
square planarsquare planar dspdsp22
trigonal bipyramidaltrigonal bipyramidal dspdsp33 or or spsp33dd
octahedraloctahedral dd22spsp33 or or spsp33dd22
Example 1Example 1: [CoF: [CoF66]]33––
Co [Ar] 3Co [Ar] 3d d 77 4 4ss22
CoCo3+3+ [Ar] 3 [Ar] 3d d 6 6
complex is paramagneticcomplex is paramagnetic
3d 4s 4p 4d
4sp3d2
octahedraloctahedral
Example 2: [Co(NH3)6]3+
Co [Ar] 3d7 4s2
Co3+ [Ar] 3d6
3d 4s 4p
complex is diamagneticcomplex is diamagnetic
4d
d2sp3
octahedraloctahedral
Example 3Example 3: [PtCl: [PtCl44]]22––, Pt, Pt2+2+ [Xe] 4 [Xe] 4f f 1414 5 5d d 8, 8,
5d 6s 6p
dsp2
complex iscomplex is DiamagneticDiamagnetic
Sq. planarSq. planar
Example 4Example 4: [NiCl: [NiCl44]]22––, Ni, Ni2+2+ [Ar] 3 [Ar] 3d d 8 8
3d 4s 4p
4sp3
complex iscomplex is paramagneticparamagnetictetrahedraltetrahedral
Example 1. called Example 1. called Outer-orbital Oh complexOuter-orbital Oh complex (Huggin) (Huggin)
Example 2. called Example 2. called Inner-orbital Oh complexInner-orbital Oh complex (Pauling) (Pauling)
Outer-orbital Oh complexes also called Outer-orbital Oh complexes also called ionic complexesionic complexes and inner-orbital and inner-orbital
complexes as the complexes as the covalent complexescovalent complexes. .
These complexes are also known as These complexes are also known as spin-freespin-free and and spin-pairedspin-paired (by Nyholom), (by Nyholom),
high-spin (HS)high-spin (HS) and and low-spin (LS)low-spin (LS) (by Orgel). (by Orgel).
Some examples:Some examples:
Inner-orbital complexesInner-orbital complexes: : d3d3-[Cr-[CrIIIIII(NH3)6]3+; (NH3)6]3+; d4-d4-[Cr[CrIIII(CN)6]4-, [Mn(CN)6]4-, [MnIIIIII(CN)6]3-; (CN)6]3-; d5-d5-
[Fe[FeIIIIII(CN)6]3-; (CN)6]3-; d6d6-[Fe-[FeIIII(CN)6]4-, [Co(CN)6]4-, [CoIIIIII(NO2)6]3-, [Co(NO2)6]3-, [CoIIIIII(H2O)6]3+, [Co(en)3]3+, (H2O)6]3+, [Co(en)3]3+,
[Pt[PtIVIV(NH3)6]4+; (NH3)6]4+; d7d7-[Co-[CoIIII(NO2)6]4-. (NO2)6]4-.
Outer-orbital complexesOuter-orbital complexes: : d3d3-[Cr-[CrIIIIII(H2O)6]3+; (H2O)6]3+; d4d4-[Cr-[CrIIII(H2O)6]2+, (H2O)6]2+,
[Mn[MnIIIIII(H2O)6]3+; (H2O)6]3+; d5d5-[Mn-[MnIIII(H2O)6]2+, [Fe(H2O)6]2+, [FeIIIIII(H2O)6]3+, [Fe(H2O)6]3+, [FeIIIIIIF6]3-; F6]3-; d6d6--
[Fe[FeIIII(H2O)6]2+, [Fe(H2O)6]2+, [FeIIII(NH3)6]2+; (NH3)6]2+; d7d7-[Co-[CoIIII(H2O)6]2+; (H2O)6]2+; d8d8- [Ni- [NiIIII(NH3)6]2+, (NH3)6]2+,
[Ni[NiIIII(H2O)6]2+; (H2O)6]2+; d9d9-[Cu-[CuIIII(NH3)6]2+; (NH3)6]2+; d10d10-[Cu-[CuII(NH3)6]1+.(NH3)6]1+.
Limitations of VBT:Limitations of VBT:
1. (a) 1. (a) Oh Oh (d2sp3 or sp3d2), (d2sp3 or sp3d2), tetrahedraltetrahedral (sp3) and (sp3) and square planarsquare planar (dsp2) (dsp2)
complexes of complexes of d1(1 unpaired electronsd1(1 unpaired electrons for Oh, Th or Sq. planar), d2 (2 for Oh, Th or Sq. planar), d2 (2
unpaired electronsunpaired electrons for Oh, Th or Sq. planar), d3 (3 unpaired electronsfor Oh, Th or Sq. planar), d3 (3 unpaired electrons for for
Oh, Th or Sq. planar) and d9 ionOh, Th or Sq. planar) and d9 ion same as d1same as d1 and hence cannot be and hence cannot be
distinguished from each other merely on the basis of the number of distinguished from each other merely on the basis of the number of
unpaired electrons (b) unpaired electrons (b) Outer-orbital Oh and Th complexesOuter-orbital Oh and Th complexes of all the ions of all the ions
viz.viz. d1 - d9d1 - d9 which have the same number of unpaired electrons cannot be which have the same number of unpaired electrons cannot be
distinguished from each other.distinguished from each other.
2. 2. Color and magnetic moments of complexes are due to d-orbital electrons.Color and magnetic moments of complexes are due to d-orbital electrons.
There must be a quantitative connection between spectra and magnetic There must be a quantitative connection between spectra and magnetic
moment. This is not revealed in VBT and moment. This is not revealed in VBT and consequently magnetic and consequently magnetic and
spectral properties could not be explained by this theoryspectral properties could not be explained by this theory..
3. VBT does not explain the behavior of complexes having 3. VBT does not explain the behavior of complexes having d8 iond8 ion (e.g. Ni+2, (e.g. Ni+2,
Pb+2, Au+3, etc.) Pb+2, Au+3, etc.) in forming 5-coordinated complexesin forming 5-coordinated complexes. Also, VBT. Also, VBT prefers prefers
only square planar geometry of complexes not Th or trigonal bipyramidal.only square planar geometry of complexes not Th or trigonal bipyramidal.
4. The metal ion has much importance while ligand is not properly stressed.4. The metal ion has much importance while ligand is not properly stressed.
5. VBT cannot explain reaction rates and mechanism of reactions.5. VBT cannot explain reaction rates and mechanism of reactions.
2.2. Crystal Field Theory and their Historical Background Crystal Field Theory and their Historical Background
(H. Bethe, L. Orgel and V. Bleck, 1935)(H. Bethe, L. Orgel and V. Bleck, 1935)
The C.F. model mainly applied for The C.F. model mainly applied for ionic crystalsionic crystals and and describedescribess the bonding the bonding
between thebetween the metal atom/ metal atom/ion and the ligandsion and the ligands ( (a –ve point charge or point a –ve point charge or point
charge for anionic Ls and point dipole/dipole for neutral moleculescharge for anionic Ls and point dipole/dipole for neutral molecules). ). This This
explainexplain the d-orbitals splitting into groups as a result of electrostatic the d-orbitals splitting into groups as a result of electrostatic
interactionsinteractions. . CFT is very useful to understand, interpret and predict the CFT is very useful to understand, interpret and predict the
magnetic behavior, colors and some structures of coordination complexes.magnetic behavior, colors and some structures of coordination complexes.
Bethe Bethe et al.et al. investigated, how the strength of a crystalline field affect the investigated, how the strength of a crystalline field affect the
electronic levels of the gaseous metal ions.electronic levels of the gaseous metal ions.
First timeFirst time, C.F. theory was developed by considering two compounds: Mn, C.F. theory was developed by considering two compounds: Mn IIIIO, O,
and Cuand CuIICl.Cl.
How does How does wewe describe and characterize the bonding between describe and characterize the bonding between M M ion and ion and
ligands in terms of ligands in terms of thisthis electronic theory? electronic theory?
Crystal field around the metal s-, p- and d-orbital (Oh):Crystal field around the metal s-, p- and d-orbital (Oh):
Basis: Basis: purely electrostatic interactionpurely electrostatic interaction
i. Spherical field: i. Spherical field: dd orbitals degenerate orbitals degenerate; ; s- and p-orbital remain unchangeds- and p-orbital remain unchanged..
ii. Unsymmetrical field: ii. Unsymmetrical field: What will happen?What will happen?
s- and p-orbitals remain degenerate even under Oh ligands field influence. s- and p-orbitals remain degenerate even under Oh ligands field influence.
However, d-orbitals split depending on ligands field type. They are:However, d-orbitals split depending on ligands field type. They are:
Diagram for the metal d-orbitals splitting in Oh Crystal FieldDiagram for the metal d-orbitals splitting in Oh Crystal Field
The energy difference between eThe energy difference between egg and t and t2g2g-orbitals in -orbitals in
the crystal field is known as the crystal field is known as crystalcrystal//ligand field ligand field
splitting energy (splitting energy (CFSE)CFSE), 10Dq or Δ, 10Dq or Δoo. . At hypothetical At hypothetical
degenerate d-orbitals, no splitting state assumed degenerate d-orbitals, no splitting state assumed
called called BarycenterBarycenter, E=0, E=0. From . From conservation of conservation of
energy statesenergy states t t2g2g orbitals lie at -0.4Δ orbitals lie at -0.4Δoo and the e and the egg
orbitals lie at +0.6Δorbitals lie at +0.6Δoo. .
((CFSECFSEohoh=-0.4nt=-0.4nt2g2g + 0.6ne + 0.6negg), n-no. of electrons), n-no. of electrons
Crystal field splitting diagramsCrystal field splitting diagrams
(i) (i) Octahedral complexOctahedral complex::
eg
t2g crystal field stabilization crystal field stabilization energy (CFSE)energy (CFSE)
Pentagonal pyramidalPentagonal pyramidal
Summary:Summary:
Crystal field stabilization energy (CFSE)Crystal field stabilization energy (CFSE)
CFSE gave CFSE gave the stability of complexthe stability of complex on placing a metal ion in the ligand field on placing a metal ion in the ligand field
generated by a set of ligands. Due to d-orbitals splitting, some of d-orbitals generated by a set of ligands. Due to d-orbitals splitting, some of d-orbitals
become lower in energy than before with respect to a spherical field known as become lower in energy than before with respect to a spherical field known as
the the barycenterbarycenter in which all five in which all five dd-orbitals are degenerate.-orbitals are degenerate. e.g. Oh complex:e.g. Oh complex:
CFSE = 0- (-4Dq) = 4Dq (CFSE = 0- (-4Dq) = 4Dq (d1-system), d1-system), d2 = 2 x 4 Dq = 8 Dq; d3 = 12Dq. For d4- d2 = 2 x 4 Dq = 8 Dq; d3 = 12Dq. For d4-
two possibilities: ttwo possibilities: t2g2g44 ( (CFSE =CFSE = 16Dq) or t16Dq) or t2g2g
33 e egg11 ( (CFSE =CFSE = 6Dq). 6Dq). As a result of As a result of
splitting, if there are any electrons occupying these orbitals, the metal ion is splitting, if there are any electrons occupying these orbitals, the metal ion is
more stable in the ligand field relative to the barycenter by an amount known more stable in the ligand field relative to the barycenter by an amount known
as the CFSEas the CFSE.. The stability will depend on ΔThe stability will depend on Δoo and also spin pairing energy (P)and also spin pairing energy (P)..
Distribution of electronsDistribution of electrons
ΔΔoo= +ve, unfavorable= +ve, unfavorable
ΔΔoo= 0 (no change in stability)= 0 (no change in stability)
ΔΔoo= -ve (gain stability)= -ve (gain stability)
d2d3
How is a How is a dd44 configuration distributed? configuration distributed?
CFSE = [-0.4xnCFSE = [-0.4xn11+0.6xn+0.6xn22] Δ] Δoo+mP+mP
Example 1Example 1: : For MnFor Mn3+ 3+ ion, the electron pairing energy, P is about 28000 cmion, the electron pairing energy, P is about 28000 cm-1-1.. ΔΔoo
values for the complexes [Mn(Hvalues for the complexes [Mn(H22O)O)66]]3+3+ and [Mn(CN) and [Mn(CN)66]]3-3- are 21000 cm are 21000 cm-1-1 and and
38500 cm38500 cm-1-1 respectively. Do these complexes have HS or LS configuration? respectively. Do these complexes have HS or LS configuration?
Also write down the configurations corresponding to these states? Also write down the configurations corresponding to these states?
Example 2Example 2:: Give the number of unpaired electrons for the [Fe(CN) Give the number of unpaired electrons for the [Fe(CN)66]]4-4- and and
[Fe(CN)[Fe(CN)66]]3-3- complexes. complexes.
dd5 – 5 – e.g., e.g., low-spinlow-spin ( ([Fe(NO2)6]3−[Fe(NO2)6]3−) has five electrons in the ) has five electrons in the tt2g2g orbitals. CFSE is 5 orbitals. CFSE is 5
x 2/5 Δx 2/5 Δoo= 2Δ= 2Δoo. . However, it is highly unfavorable condition due to the greatest However, it is highly unfavorable condition due to the greatest
loss of exchange energyloss of exchange energy. In . In high-spinhigh-spin ( ([FeBr6]3−[FeBr6]3−), CFSE is (3 x 2/5 Δ), CFSE is (3 x 2/5 Δoo) - (2 x 3/5 ) - (2 x 3/5
ΔΔoo) = 0. The stabilization generated by the electrons in the lower orbitals is ) = 0. The stabilization generated by the electrons in the lower orbitals is
canceled out by the destabilizing effect of the electrons in the upper orbitals.canceled out by the destabilizing effect of the electrons in the upper orbitals.
d6d6- CFSE = 2.4Δ- CFSE = 2.4Δo o (LS) and if consider pairing energy (P), 2.4Δ(LS) and if consider pairing energy (P), 2.4Δoo- 3P. In HS case, - 3P. In HS case,
CFSE = 0.4ΔCFSE = 0.4Δoo and 0.4Δ and 0.4Δoo-P. -P. d7d7- 1.8Δ- 1.8Δoo (LS)/1.8Δ (LS)/1.8Δoo–3P & 0.8Δ–3P & 0.8Δoo(HS)/ 0.8Δ(HS)/ 0.8Δoo-2P. -2P.
Similarly,d8, d9 and d10. Similarly,d8, d9 and d10.
Crystal Field stabilization is applicable to metal complexes of all Crystal Field stabilization is applicable to metal complexes of all geometries, ,
inluding square-planar inluding square-planar dd8 complexes having very large CFSE.8 complexes having very large CFSE.
High-spin (HS-) and low-spin (LS-) Oh complexesHigh-spin (HS-) and low-spin (LS-) Oh complexes
Ligands cause large Δ for Ligands cause large Δ for d-orbitals are known as are known as strong-field ligandsstrong-field ligands. e.g. . e.g.
CN−, NO2-, and CO, produce CN−, NO2-, and CO, produce low-spinlow-spin complexes and follow complexes and follow Aufbau principleAufbau principle..
Conversely, ligands (e.g., I− & Br−) cause small Δ for Conversely, ligands (e.g., I− & Br−) cause small Δ for dd-orbitals are known as -orbitals are known as
weak-field ligands weak-field ligands and produce and produce High-spinHigh-spin complexes and follow complexes and follow Hund’s ruleHund’s rule..
Factors affecting the magnitude of Factors affecting the magnitude of ΔΔoo 1. 1. Oxidation state of the metal ionOxidation state of the metal ion: e.g., [Co(H2O)6]+3; : e.g., [Co(H2O)6]+3; ΔΔoo=18,600 cm-1 =18,600 cm-1
and and [Co(H2O)6]+2; [Co(H2O)6]+2; ΔΔoo=9300 cm-1=9300 cm-1
2. 2. Nature of the metal ionNature of the metal ion: : Δ increases (30-50%) from 3d to 4d to 5d in Δ increases (30-50%) from 3d to 4d to 5d in
the same oxidation state.the same oxidation state.
3. 3. Number and geometry of the ligandsNumber and geometry of the ligands: e.g., : e.g., ΔΔt t = - 4/9 Δ= - 4/9 Δo o smaller than smaller than
Oh complexes and so ΔOh complexes and so Δtt < P gave HS-complexes. < P gave HS-complexes.
4. 4. Nature of the ligandsNature of the ligands: Spectrochemical series (see).: Spectrochemical series (see).
THE THE SPECTROCHEMICAL SERIESSPECTROCHEMICAL SERIES (Tsuchida 1938) (Tsuchida 1938) Based on factors affecting CFSE, Based on factors affecting CFSE, OO like: like:
i.i. the nature of the metal ion. the nature of the metal ion. ii.ii. the metal oxidation state. the metal oxidation state. iii.iii. the arrangement of the arrangement of
the ligands around the metal ion. the ligands around the metal ion. iv.iv. the nature of the ligands surrounding the the nature of the ligands surrounding the
metal ion. metal ion.
Tsuchida experimentally saw the effect of different metal oxidation state and the Tsuchida experimentally saw the effect of different metal oxidation state and the
ligands in the CFSE determination. The arrangement of metal ions or ligands ligands in the CFSE determination. The arrangement of metal ions or ligands
from higher to lower or vice versa is called from higher to lower or vice versa is called spectrochemical seriesspectrochemical series. e.g.,. e.g.,
A.A. When the When the geometry and the ligandsgeometry and the ligands are held constant, splitting decreases in are held constant, splitting decreases in
the following order:the following order: strong-field ions strong-field ions PtPt4+ 4+ >Ir>Ir3+ 3+ >Rh>Rh3+ 3+ >Co>Co3+ 3+ >Cr>Cr3+ 3+ >Fe>Fe3+ 3+ >Fe>Fe2+ 2+ >Co>Co2+ 2+
>Ni>Ni2+ 2+ >Mn>Mn2+2+ weak-field ions weak-field ions
B.B. When the When the geometrygeometry and the and the metalmetal are held constant, are held constant, dd orbitals splitting orbitals splitting
decreases in the following order:decreases in the following order: weak-field ligands weak-field ligands I− < < Br− < < S2− < < SCN− < < Cl− < <
NO3− < < N3− < < F− < < OH− < < C2O42− < < H2O < < NCS− < < CH3CN < < py < < NH3 < < en < < 2,2'-bipyridine
< < phen < < NO2− < < PPh3 < < CN− < < CO strong-field ligandsstrong-field ligands
The pairing energy (P) and CFSE
Pairing energy (P) vs. Pairing energy (P) vs. OO
1. If 1. If OO < P, weak field; e.g., < P, weak field; e.g., [Cr(H[Cr(H22O)O)66]]2+2+
2. If 2. If OO > P, strong field; e.g., > P, strong field; e.g., [Cr(CN)[Cr(CN)66]]44––
3. If 3. If OO = P, ??; e.g., d4, d6 & d8. = P, ??; e.g., d4, d6 & d8.
3Distribution of d-electrons in t3Distribution of d-electrons in t2g2g- and e- and egg-sets either in strong or weak Oh fields -sets either in strong or weak Oh fields
are same for are same for d1, d2 & d3 ionsd1, d2 & d3 ions. Stronger fields (Δo >P, . Stronger fields (Δo >P, d4, d5, d6 & d7 ionsd4, d5, d6 & d7 ions) have ) have
electrons in telectrons in t2g2g ( (low spin/spin pairedlow spin/spin paired and so lower resultant spin value), Weaker and so lower resultant spin value), Weaker
fields (Δo <P, fields (Δo <P, d4, d5, d6 & d7 ionsd4, d5, d6 & d7 ions) have in e) have in egg– electrons (– electrons (high spin/spin freehigh spin/spin free
and so greater resultant spin value). and so greater resultant spin value). d8, d9 & d10d8, d9 & d10 stronger and weaker fields stronger and weaker fields
have same distributions in thave same distributions in t2g2g and e and egg. .
e.g., LS- and HS-Oh complexes-
dx config.
examples P value (cm-1)
Δo value (cm-1)
spin stateCFT predicted
Observed expt.
Relative magnitude of Δo and P
d4 [Cr(H2O)6]2+[Mn(H2O)6]3+
2350028000
1390021000
HSHS
HSHS
Δo <PΔo <P
d5 [Mn(H2O)6]2+[Fe(H2O)6]3+
2550030000
780013700
HSHS
HSHS
Δo <PΔo <P
d6 [Fe(H2O)6]2+[Fe(CN)6]4-[Co(NH3)6]3+[CoF6]3-
17600176002100021000
10400330003200013000
HSLSLSHS
HSLSLSHS
Δo <PΔo >PΔo >PΔo <P
d7 [Co(H2O)6]2+ 22500 93000 HS HS Δo <P
Splitting in Tetrahedral geometrySplitting in Tetrahedral geometry: 4-coordinate complexes, e.g., : 4-coordinate complexes, e.g., Th Th & & Sq. planarSq. planar..
The CFSE is low and unable to force electrons to pair-The CFSE is low and unable to force electrons to pair-HS complexesHS complexes result. result.
2.What will happen with strong field ligands?2.What will happen with strong field ligands? 1. Why1. Why ΔΔt t = 0.45Δ= 0.45Δoo??
et2
d-electrons config. in Td-electrons config. in Thh- HS and LS-ligand fields: - HS and LS-ligand fields: CFSE = 0-(- 6Dq) = 6Dq for CFSE = 0-(- 6Dq) = 6Dq for dd11
system; n= no. of unpaired es.system; n= no. of unpaired es.
ddxx config. config.Weak field (HS-Weak field (HS-
complexes)complexes)Strong field (LS-Strong field (LS-
complexes)complexes)
tt2g 2g pp e egg
qq config. config. nn tt2g 2g pp e egg
qq config. config. nn
dd11
dd22
tt2g 2g 00 e egg
11
tt2g 2g 00 e egg
22
1122
tt2g 2g 00 e egg
11
tt2g 2g 00 e egg
22
1122
dd33
dd44
dd55
dd66
tt2g 2g 11 e egg
22
tt2g 2g 22 e egg
22
tt2g 2g 33 e egg
22
tt2g 2g 33 e egg
33
33445544
tt2g 2g 00 e egg
33
tt2g 2g 00 e egg
44
tt2g 2g 11 e egg
44
tt2g 2g 22 e egg
44
11001122
dd77
dd88
dd99
dd1010
tt2g 2g 33 e egg
44
tt2g 2g 44 e egg
44
tt2g 2g 55 e egg
44
tt2g 2g 66 e egg
44
33221100
tt2g 2g 33 e egg
44
tt2g 2g 44 e egg
44
tt2g 2g 55 e egg
44
tt2g 2g 66 e egg
44
33221100
SP
Why ΔWhy Δspsp = 1.3Δ = 1.3Δoo??z out conditionz out condition
Sq. planarSq. planar
M-ions (dM-ions (d88)-strong ligand field gave Sq. planar (LS) complexes. e.g., [Ni(CN))-strong ligand field gave Sq. planar (LS) complexes. e.g., [Ni(CN)44]]-2-2, ,
[Pt/PdCl[Pt/PdCl44]]-2-2, [Pt(NH, [Pt(NH33))44]]+2+2 & [AuCl & [AuCl44]]-1-1 where d where dx2-y2x2-y2 remain always unoccupied. remain always unoccupied.
Cu+2Ca+2 Sc+2 Ti+2V+2 Cr+2 Mn+2 Fe+2 Co+2 Ni+2 Zn+2
0.6
1.0
Å
Ioni
c ra
dii
no. of 3d-electrons
3d=0 3d=10
(O(Ohh ionic radii of M ionic radii of M+2+2 for 1 for 1stst row transition metals) row transition metals)
For MFor M+3+3 ions trends and explanations are same. ions trends and explanations are same.
Sc+2-unstableSc+2-unstable
CFSE= - ve CFSE= - ve (more –ve values, more stable complexes) (more –ve values, more stable complexes)
Applications of C.F. Theory
Ionic RadiiIonic Radii::
11stst row TM, ionic radii of M row TM, ionic radii of M+2+2-ions, e.g., metals halides (MX-ions, e.g., metals halides (MX22, X= F, Cl, Br, , X= F, Cl, Br,
I, Oh shape). I, Oh shape). Theoretical and Experimental values are sameTheoretical and Experimental values are same for for CaCa+2+2 (d (d00), ),
MnMn+2+2 (d (d55, HS), Zn, HS), Zn+2+2 (d (d1010), CFSE = 0, Two extreme cases, V), CFSE = 0, Two extreme cases, V2+2+ (d (d33) & Ni) & Ni2+2+ (d (d88) )
in a weak ligand fields (Xs), CFSE = 1.2in a weak ligand fields (Xs), CFSE = 1.2ΔΔoo, others (d, others (d22, d, d44, d, d77 & d & d99), CFSE = ), CFSE =
0.6 - 0.80.6 - 0.8ΔΔo. o. Why? Why? FF-->Cl>Cl-->Br>Br-->I>I-- in electronegativity in electronegativity, , What will be the ionic What will be the ionic
radii trend in 1radii trend in 1stst TM? TM?
Strong ligands?Strong ligands? Ionic radii decreases for the strong field case until the Ionic radii decreases for the strong field case until the
tt2g2g66 config. is reached, due to increasing nuclear charge and poor config. is reached, due to increasing nuclear charge and poor
shielding by tshielding by t2g2g d-electrons. At this point the next electron enters the e d-electrons. At this point the next electron enters the egg--
orbital directed at the ligands, repellingorbital directed at the ligands, repelling them and causing an increase in them and causing an increase in
the effective radius of the metal-ligand.the effective radius of the metal-ligand.
Hydration enthalpy/ stability of complexesHydration enthalpy/ stability of complexes
Cu+2Ca+2 Sc+2 Ti+2V+2 Cr+2 Mn+2 Fe+2 Co+2 Ni+2 Zn+2
350
400
450
500
heat
s of
hyd
rati
on(k
cal/
mol
)
xx
xx
xx
xx
experimental values of CFSE
Calculated values of CFSE
Cu+3Ca+2 Sc+3 Ti+3V+3 Cr+3 Mn+3 Fe+3 Co+3 Ni+3 Zn+3
700
800
900
1000
heat
s of
hyd
rati
on(k
cal/
mol
)
xx
x
xx
x
x
experimental values of CFSE
Calculated values of CFSE
CFSECFSE value for Heat of hydration of Mvalue for Heat of hydration of M+2+2 and M and M+3+3-ions of 1-ions of 1stst-row TM, a similar -row TM, a similar
trends is found for the lattice energy trends is found for the lattice energy vsvs TM-ions. TM-ions. Higher the heat of hydration Higher the heat of hydration
or lattice energy-more stable compounds (CFSE?)or lattice energy-more stable compounds (CFSE?). .
(Hydration enthalpy of M(Hydration enthalpy of M+2+2 and and MM+3+3 for 1for 1stst row TMs in row TMs in Oh complexesOh complexes))
Here, the experimental values increasing irregularly, Here, the experimental values increasing irregularly, maxima at V+2 (d3 ion) maxima at V+2 (d3 ion)
and Ni+2 (d8) and minima at Ca+2(d0 ion), Mn+2 (d5) and Zn+2 (d10).and Ni+2 (d8) and minima at Ca+2(d0 ion), Mn+2 (d5) and Zn+2 (d10). The The
unexpected maxima and minima can be explained on the basis of CFSE unexpected maxima and minima can be explained on the basis of CFSE
concept. [M(H2O)6]+2 are high-spin Oh complexes and for high-spin concept. [M(H2O)6]+2 are high-spin Oh complexes and for high-spin
complexes, CFSE is minimum (zero) for d0 (Ca+2), d5 (Mn+2) and d10 (Zn+2) complexes, CFSE is minimum (zero) for d0 (Ca+2), d5 (Mn+2) and d10 (Zn+2)
ions and maximum (=1.2 Δo) for d3 (V+2) and d8 (Ni+2) ions. ions and maximum (=1.2 Δo) for d3 (V+2) and d8 (Ni+2) ions.
In totoIn toto, Ca+2/+3 to Zn+2/+3 ionic radii decrease-hydration energy increase., Ca+2/+3 to Zn+2/+3 ionic radii decrease-hydration energy increase.
Color of ComplexesColor of Complexes
The bright colors exhibited by many The bright colors exhibited by many coordination compounds can be explained by can be explained by
C.F. Theory. C.F. Theory.
When white light is allowed to fall on a complex, the following things may occur:When white light is allowed to fall on a complex, the following things may occur:
i. The complex may i. The complex may absorbed theabsorbed the whole white lightwhole white light. Thus complex appears . Thus complex appears
blackblack..
ii. The complex may ii. The complex may reflect (or transmit) the whole lightreflect (or transmit) the whole light. In this case it appears . In this case it appears
whitewhite..
iii. The complex may iii. The complex may absorb some of it and may reflect (or transmit) the absorb some of it and may reflect (or transmit) the
remaining lightremaining light. In this case the complex has some color, i.e. . In this case the complex has some color, i.e. it is coloredit is colored. .
The absorption of light by the complexes takes place in the The absorption of light by the complexes takes place in the visible region of visible region of
the spectrumthe spectrum ( (4000Ǻ to 7000Ǻ wavelength)4000Ǻ to 7000Ǻ wavelength). .
High----------------------decreasing energy-----------------------------------------→Low
Color absorbed
Violet Blue Green-blue
Blue-green
Green Yellow-green
Yellow Orange Red
λ of the absorbed
4000Å 4350 Å
4800 Å 4900 Å
5000 Å
5600 Å 5800 Å 5900 Å 6050-7000 Å
Low ------------------increasing wavelength--------------------------------------------→High
Color transmitted (color of the complex
Yellow-green
Yellow Orange Red Purple Violet Blue Green Blue-green
[Cu(H[Cu(H22O)O)44]]2+2+ ions----hydrated cupric sulphate (blue), [Ti(H ions----hydrated cupric sulphate (blue), [Ti(H22O)O)66]]3+3+ ions (purple) ions (purple)
The color of the absorbed light is different from that of the transmitted light The color of the absorbed light is different from that of the transmitted light
called called complementary colorcomplementary color. The relation between the colors of the absorbed . The relation between the colors of the absorbed
and reflected light is as below:and reflected light is as below:
The complex ions absorb light in the The complex ions absorb light in the IR (λ> 7000 Å)IR (λ> 7000 Å) or or UV (λ< 4000 Å)UV (λ< 4000 Å) are are
colorless, e.g. (i) anhydrous cupric sulphate is colorless, IR region. (ii) colorless, e.g. (i) anhydrous cupric sulphate is colorless, IR region. (ii)
[Cu(CN)[Cu(CN)44]]2-2- ion UV region, colorless. ion UV region, colorless.
Wave no. (ν) = 1/ λ(cm) =1/ Å x 10Wave no. (ν) = 1/ λ(cm) =1/ Å x 10-8-8 cm =x cm cm =x cm-1-1 (1 cm (1 cm-1-1=2.85 x 10=2.85 x 10-3-3 kcal/mole or kcal/mole or
350 cm350 cm-1-1 = 1.0 kcal/mole). = 1.0 kcal/mole).
Rarely, the energy of the photon absorbed corresponds exactly to the size of Rarely, the energy of the photon absorbed corresponds exactly to the size of
the gap Δ; Other factors (such as electron-electron repulsion and the gap Δ; Other factors (such as electron-electron repulsion and
Jahn-Teller effects) that also affect the energy difference between the ) that also affect the energy difference between the
ground and excited states.ground and excited states.
Mol
ar a
bsor
ptan
ce
30000 20000
(cm-1)
10000
Å= 5000
(Visible absorption spectrum of [Ti(H(Visible absorption spectrum of [Ti(H22O)O)66]]3+3+ ion. ion.
Peak of the curve shows the maximum absorption)Peak of the curve shows the maximum absorption)
Visible absorption spectrum of a complex ion is useful in predicting the color of Visible absorption spectrum of a complex ion is useful in predicting the color of
the complex. e. g., [Ti(Hthe complex. e. g., [Ti(H22O)O)66]]3+3+ ion shows absorption maxima at a wavelength of ion shows absorption maxima at a wavelength of
about about λλ- 5000Ǻ- 5000Ǻ. . νν = 1/ = 1/ λλ (in cm). 5000Ǻ- green color absorbed. Transmitted light (in cm). 5000Ǻ- green color absorbed. Transmitted light
is purple (Δo, E= 57kcal/mole), corresponding to tis purple (Δo, E= 57kcal/mole), corresponding to t2g2g1 1 eegg
00------t------t2g2g0 0 eegg
1 1 called called d-d or d-d or
ligand field transitionligand field transition..
Magnetism of ComplexesMagnetism of Complexes
dx config. Ions n µs µexp
d1d2d3d4d5d6d7d8d9
Ti+3Ti+2, V+2V+2 Cr+3
Cr+2 Mn+3Mn+2 Fe+3Fe+2 Co+3
Co+2Ni+2Cu+2
123454321
1.732.833.874.905.924.903.872.831.73
1.73-1.852.75-2.853.80-3.904.75-4.905.65-6.105.10-5.704.30-5.202.80-4.001.70-2.20
µµss = √ n(n+2) BM. (BM- Bohr Magneton). = √ n(n+2) BM. (BM- Bohr Magneton). Spin only formula is valid in 1Spin only formula is valid in 1stst row TMs row TMs. .
[CoCl[CoCl44]]2-2- & [MnF & [MnF44]]2-2- gave 1.73 and 5.9BM respectively, predict the geometry? gave 1.73 and 5.9BM respectively, predict the geometry?
Magnetic property of coordination compounds gave:Magnetic property of coordination compounds gave:
• Unpaired electrons in d-orbitalsUnpaired electrons in d-orbitals and the magnetic momentand the magnetic moment
• Transition metal complexes are Paramagnetic or Diamagnetic. Transition metal complexes are Paramagnetic or Diamagnetic.
• The crystal field splitting diagram as well as strong or weak field ligands The crystal field splitting diagram as well as strong or weak field ligands
and the way of the ligand field splitting parameter changes with the nature and the way of the ligand field splitting parameter changes with the nature
and oxidation state of a transition element (HS and LS complexes).and oxidation state of a transition element (HS and LS complexes).
Distortion of Oh Complexes Distortion of Oh Complexes
In Oh-complexes, when all In Oh-complexes, when all ligand electron clouds and metal ion are at the same ligand electron clouds and metal ion are at the same
length called regular (i.e. symmetrical) Oh complexeslength called regular (i.e. symmetrical) Oh complexes. Conversely, unequal . Conversely, unequal
length Oh arelength Oh are called called distorted Oh complexesdistorted Oh complexes. The change in shape is called . The change in shape is called
distortiondistortion..
Distorted in Oh complexes may be of the following three types:Distorted in Oh complexes may be of the following three types:
(i) Diagonally distorted Oh complexes: Distortion of a regular Oh along two-fold (i) Diagonally distorted Oh complexes: Distortion of a regular Oh along two-fold
axis.axis.
(ii) Trigonally distorted Oh complexes: Distortion along a three-fold axis. (ii) Trigonally distorted Oh complexes: Distortion along a three-fold axis.
(iii) (iii) Tetragonally distorted Oh complexesTetragonally distorted Oh complexes: Distortion along a four-fold axis. : Distortion along a four-fold axis.
For example: (a) For example: (a) Elongation along z-axis (Z-out condition):Elongation along z-axis (Z-out condition): two long bonds (z- two long bonds (z-
axis) and four short bonds (xy plane). e.g., axis) and four short bonds (xy plane). e.g.,
i. CuCli. CuCl22: four at 2.30 Å and two at 2.95 Å bond length.: four at 2.30 Å and two at 2.95 Å bond length.
ii. CuFii. CuF22: four at 1.93 Å and two at 2.27 Å bond length.: four at 1.93 Å and two at 2.27 Å bond length.
iii. LS-Oh complexes of Ni+2, Pd+2 and Pt+2 (iii. LS-Oh complexes of Ni+2, Pd+2 and Pt+2 (all d8 ionsall d8 ions) - strong distortion gave ) - strong distortion gave
square planar geometry .square planar geometry .
(b) (b) Compression along z-axis (Z-in condition)Compression along z-axis (Z-in condition):: two short bonds (z axis) and four two short bonds (z axis) and four
long bonds (xy plane). e.g., (i) Klong bonds (xy plane). e.g., (i) K22CuFCuF44: two at 1.95 Å and four at 2.08 Å. (ii) FeF: two at 1.95 Å and four at 2.08 Å. (ii) FeF22: :
two at 1.99 Å and four at 3.12 Å. two at 1.99 Å and four at 3.12 Å.
(conditions (a) & (b) gave Tetragonal distortion geometry) shown below:(conditions (a) & (b) gave Tetragonal distortion geometry) shown below:
Tetrahedral
x2-y2 z2 eg
xy yz xz t2g
-0.6 t
+0.4 tt = 0.45 o
t2g + eg
xy yz xz x2-y2 z2
degenerate d-orbitals on Mn+
x2-y2 z2
xy yz xz
eg
t2g
+6Dq
-4Dq
10Dq
Octahedral
x2-y2
z2
xy
yz xz
Tetragonal
x2-y2
xy
z2
yz xz
Sq. Planar
12
3sp
(z-elongation)
No of d No of d
electronselectrons
HS-Oh complexes: weak ligand field HS-Oh complexes: weak ligand field LS-Oh complexes: strong ligand fieldLS-Oh complexes: strong ligand field
Distribution of es- in t2g Distribution of es- in t2g
and eg-orbitalsand eg-orbitals
Predicted Predicted
DistortionDistortion
Distribution of es- in t2g Distribution of es- in t2g
and eg-orbitalsand eg-orbitals
Predicted Predicted
DistortionDistortion
d0d0
d1d1
d2d2
d3d3
d4d4
d5d5
d6d6
d7d7
d8d8
d9d9
d10d10
tt2g2g00(sym)-e(sym)-e
gg00(sym)(sym)
tt2g2g11(unsym)-e(unsym)-e
gg00(sym)(sym)
tt2g2g22(unsym)-e(unsym)-e
gg00(sym)(sym)
tt2g2g33(sym)-e(sym)-e
gg00(sym)(sym)
tt2g2g33(sym)-e(sym)-e
gg11(unsym)(unsym)
tt2g2g33(sym)-e(sym)-e
gg22(sym)(sym)
tt2g2g44(unsym)-e(unsym)-e
gg22(sym)(sym)
tt2g2g55(unsym)-e(unsym)-e
gg22(sym)(sym)
tt2g2g66(sym)-e(sym)-e
gg22(sym)(sym)
tt2g2g66(sym)-e(sym)-e
gg33(unsym)(unsym)
tt2g2g66(sym)-e(sym)-e
gg44(sym)(sym)
No distor.No distor.
Slight DSlight D
Slight DSlight D
No distor.No distor.
Strong DStrong D
No distor.No distor.
Slight DSlight D
Slight DSlight D
No distor.No distor.
Strong DStrong D
No distor.No distor.
tt2g2g00(sym)-e(sym)-e
gg00(sym)(sym)
tt2g2g11(unsym)-e(unsym)-e
gg00(sym)(sym)
tt2g2g22(unsym)-e(unsym)-e
gg00(sym(sym))
tt2g2g33(sym)-e(sym)-e
gg00(sym)(sym)
tt2g2g44(unsym)-e(unsym)-e
gg00(sym)(sym)
tt2g2g55(unsym)-e(unsym)-e
gg00(sym)(sym)
tt2g2g66(sym)-e(sym)-e
gg00(sym)(sym)
tt2g2g66(sym)-e(sym)-e
gg11(unsym)(unsym)
tt2g2g66(sym)-e(sym)-e
gg22(unsym)(unsym)
tt2g2g66(sym)-e(sym)-e
gg33(unsym)(unsym)
tt2g2g66(sym)-e(sym)-e
gg44(sym)(sym)
No distor.No distor.
Slight DSlight D
Slight DSlight D
No distor.No distor.
Slight DSlight D
Slight DSlight D
No distor.No distor.
Strong DStrong D
Strong Strong
Distor. leads Distor. leads
sq. planarsq. planar
Strong DStrong D
No distor. No distor.
No Distortion ConditionNo Distortion Condition: Both t: Both t2g2g and e and egg-sets as symmetrical orbitals lead to -sets as symmetrical orbitals lead to
perfectly symmetrical (regular) Oh complexes. perfectly symmetrical (regular) Oh complexes.
Condition for slight DistortionCondition for slight Distortion: d-orbitals of Oh complexes have : d-orbitals of Oh complexes have tt2g2g-orbitals as -orbitals as
unsymmetrical orbitals gave slight distortion in the complexunsymmetrical orbitals gave slight distortion in the complex, because they do , because they do
not come directly in front of the ligands approaching for metal ion. not come directly in front of the ligands approaching for metal ion. Only slight Only slight
distortions from the regular Oh was observed. However, not experimentally distortions from the regular Oh was observed. However, not experimentally
detected in Oh.detected in Oh.
Condition for strong DistortionCondition for strong Distortion: e: egg orbitals which point directly towards the orbitals which point directly towards the
ligands and are unsymmetrical i.e. contain 1, 3 and 2 electrons (ligands and are unsymmetrical i.e. contain 1, 3 and 2 electrons (only in LS-only in LS-
complexescomplexes), gave strong distortions, leading to tetragonal and even to ), gave strong distortions, leading to tetragonal and even to sq. sq.
planar complexesplanar complexes..
Jahn-Teller Effect/ Theorem (1937)Jahn-Teller Effect/ Theorem (1937): Explain, why certain complexes undergo : Explain, why certain complexes undergo
distortion to assume distorted geometry?distortion to assume distorted geometry? (i.e. (i.e. Oh to tetragonalOh to tetragonal))
StatementStatement “ “any non-linear molecular system possessing degenerate electronic any non-linear molecular system possessing degenerate electronic
state will be unstable and will undergo distortion to form a system of lower state will be unstable and will undergo distortion to form a system of lower
symmetry and lower energy and thus will remove degeneracysymmetry and lower energy and thus will remove degeneracy.” .” Jahn-Teller Jahn-Teller
distortion is automatic for the non-linear molecular systems.distortion is automatic for the non-linear molecular systems.
It does not predict the nature or its magnitude of distortion. However, always It does not predict the nature or its magnitude of distortion. However, always
occurs in a manner which decrease in the energy of the system.occurs in a manner which decrease in the energy of the system.
SymmetricalSymmetrical: : tt2g2g-orbitals-orbitals: t: t2g2g00, t, t2g2g
33, t, t2g2g66; ; eegg-orbitals-orbitals: e: egg
00, e, egg44 and e and egg
22 in HS-complexes in HS-complexes
(d(dxx2-2-yy2)2)11 (d (dzz2)2)1 1 ((No J-T distortion observedNo J-T distortion observed).).
UnsymmetricalUnsymmetrical: : tt2g2g-orbitals-orbitals: t: t2g2g11, t, t2g2g
22, t, t2g2g44, t, t2g2g
55; ; eegg-orbitals-orbitals: e: egg
11, e, egg33 and e and egg
22 in LS- in LS-
complexes (dcomplexes (dxx22-y-y2)2)00 (d (dzz2)2)22 ((J-T distortion observedJ-T distortion observed).).
Cause of distortion with some complexesCause of distortion with some complexes: e.g., (i) : e.g., (i) dd44 ion ion (HS): two possible (HS): two possible
config. of electrons in tconfig. of electrons in t2g2g- & e- & egg-orbitals: 1. t-orbitals: 1. t2g2g33 d dzz2211 d dxx22-y-y220 0 & 2. t& 2. t2g2g
33 d dzz2200 d dxx22-y-y2211
case 1case 1, M, M++-L interaction along the z-axis is less than along the x- and y-axes, -L interaction along the z-axis is less than along the x- and y-axes,
leads to a larger inter-ionic distance along the z-axis and hence a tetragonal leads to a larger inter-ionic distance along the z-axis and hence a tetragonal
structure (structure (called z out condition or z-elongationcalled z out condition or z-elongation). ).
(ii) (ii) dd99 ion ion (Cu (Cu2+ 2+ complex) e.g., aq. soln. of [Cu(NHcomplex) e.g., aq. soln. of [Cu(NH33))44]]2+2+. The tetragonal distortion is . The tetragonal distortion is
so large that sq. planar complex results. M-ion has config. tso large that sq. planar complex results. M-ion has config. t2g2g66eegg
33 in both the in both the
fields (LS & HS). The two possible arrangements of electrons in tfields (LS & HS). The two possible arrangements of electrons in t2g2g- and e- and egg--
orbitals are: 1. torbitals are: 1. t2g2g66 d dzz2222 d dxx22-y-y2211 and 2. t and 2. t2g2g
66 d dzz2211 d dxx22-y-y2222
Thus, a large distortion/Asymmetry is only due to eThus, a large distortion/Asymmetry is only due to egg incomplete orbitals. incomplete orbitals.
Explanation of DistortionExplanation of Distortion: Strong distortion mainly due to repulsion of ligands : Strong distortion mainly due to repulsion of ligands
by the electrons in eby the electrons in egg-orbitals. In -orbitals. In dd44 ion ion (HS) and (HS) and dd99 ion (Cu ion (Cu+2+2 ion, LS & HS ion, LS & HS), ), case case
11 showed the d-electron charge density becomes higher in z direction than x and showed the d-electron charge density becomes higher in z direction than x and
y direction. The dy direction. The dzz2 orbital is 2 orbital is half filled (dhalf filled (d44) & completely filled) & completely filled and points at the and points at the
ligands on the z-axis offers greater shielding of the Culigands on the z-axis offers greater shielding of the Cu+2+2 nucleus than the half nucleus than the half
filled dfilled dxx2-2-yy2 orbital, which points towards the ligands in the x, y-axes. The 2 orbital, which points towards the ligands in the x, y-axes. The
ligands on the x- and y-axes experience a higher effective nuclear charge, while ligands on the x- and y-axes experience a higher effective nuclear charge, while
those on z-axis experience a lower effective nuclear charge. So, the ligands on those on z-axis experience a lower effective nuclear charge. So, the ligands on
the x- and y-axes are drawn in closer to the Cuthe x- and y-axes are drawn in closer to the Cu+2+2 nucleus and those on the z-axis nucleus and those on the z-axis
move further out (move further out (called z out condition or z-elongationcalled z out condition or z-elongation). ).
Thus, the Oh-complex will distorted to tetragonal geometry which is elongated Thus, the Oh-complex will distorted to tetragonal geometry which is elongated
along z direction and compressed along x and y directionsalong z direction and compressed along x and y directions. Since the distortion . Since the distortion
to tetragonal geometry is automatic (without supplying energy from outside), the to tetragonal geometry is automatic (without supplying energy from outside), the
overall energy of the unsplit eoverall energy of the unsplit egg orbitals are zero. orbitals are zero. Therefore, sum of energy after Therefore, sum of energy after
splitting must be zero.splitting must be zero. ConclusionConclusion: whenever there are more electrons in d: whenever there are more electrons in dzz2 2
orbital than in dorbital than in dxx2-2-yy2 orbital (Oh complex) of any M-ion distortion follow the above 2 orbital (Oh complex) of any M-ion distortion follow the above
rule and maintain the centre of gravity rule. rule and maintain the centre of gravity rule.
Consider configuration 2Consider configuration 2: (d: (dzz2)2)11 (d (dxx22-y-y2)2)22 t t2g2g66, explanation for this observation is , explanation for this observation is
exactly the same. However, dexactly the same. However, dxx2-2-yy2 will be higher in energy. The resulting 2 will be higher in energy. The resulting
distortion will be called distortion will be called z in conditionz in condition. . How then to decide which of the two How then to decide which of the two
possible Oh distortion config.: 1. (dpossible Oh distortion config.: 1. (dzz2)2)22 (d (dxx2-2-yy2)2)1 1 tt2g2g66 and 2. t and 2. t2g2g
66 (d (dzz2)2)11 (d (dxx2-2-yy2)2)22 would would
yield the more stable complex?yield the more stable complex?
CFT offers no way of deciding itCFT offers no way of deciding it. . Experimental results, however, show that it is Experimental results, however, show that it is z z
out conditionout condition Oh distortion configuration with two long and four short bonds Oh distortion configuration with two long and four short bonds
which is more stablewhich is more stable. There is no theoretical explanation for the instability of . There is no theoretical explanation for the instability of
structure corresponding to structure corresponding to z in conditionz in condition having four long and two short bonds. having four long and two short bonds.
tt2g2g11 configuration in Oh complexes configuration in Oh complexes: Single electron can occupy any of the three : Single electron can occupy any of the three
tt2g2g orbitals. If the electron is in d orbitals. If the electron is in dxyxy orbital, it would screen the M-ion nucleus orbital, it would screen the M-ion nucleus more more
effectively in xy plane than in xz & yz planes. This would reduce the attraction effectively in xy plane than in xz & yz planes. This would reduce the attraction
between M-L in xy plane. So, the Oh geometry of the complex would get between M-L in xy plane. So, the Oh geometry of the complex would get
distorted by the elongation of the M-L bonds in xy plane. Repulsion increased distorted by the elongation of the M-L bonds in xy plane. Repulsion increased
and energy decreased in the same plane as compared to yz & xz planes. and energy decreased in the same plane as compared to yz & xz planes.
On the other handOn the other hand, if the electron is present in either d, if the electron is present in either dxzxz or d or dyzyz orbital, it will orbital, it will
screen M-ion nucleusscreen M-ion nucleus more effectively in the xz and yz planes compared to the more effectively in the xz and yz planes compared to the
xy plane, and decrease attraction between M-L along the z axis. Thus, the bonds xy plane, and decrease attraction between M-L along the z axis. Thus, the bonds
along the z direction elongated and get distorted to tetragonal geometry.along the z direction elongated and get distorted to tetragonal geometry.
The energies of both dThe energies of both dxyxy and d and dxx2-2-yy2 orbitals (being functions of the same 2 orbitals (being functions of the same
variables x & y) alter in a similar manner due to distortion of Oh geometry by variables x & y) alter in a similar manner due to distortion of Oh geometry by
Jahn-Tellar effect. The change in energies of these orbitals will again obey the Jahn-Tellar effect. The change in energies of these orbitals will again obey the
centre of gravity rule. centre of gravity rule.
The J-T effect shown by tThe J-T effect shown by t2g2g orbitals is much weaker than e orbitals is much weaker than eg g orbitals. Because in orbitals. Because in
tt2g2g orbitals the charge density lie in between the x, y and z directions and not orbitals the charge density lie in between the x, y and z directions and not
directly in the x, y and z directions along which the ligands are placed whereas directly in the x, y and z directions along which the ligands are placed whereas
in ein egg orbitals, the charge density lie directly in the directions along which the orbitals, the charge density lie directly in the directions along which the
ligands are placed. ligands are placed.
Thus, an electron in any of the tThus, an electron in any of the t2g2g orbitals would shield the positive charge of orbitals would shield the positive charge of
M-ion much less effectively in the x, y and z directions, i.e. along the directions M-ion much less effectively in the x, y and z directions, i.e. along the directions
of the ligands than the electron placed in any of the eof the ligands than the electron placed in any of the egg orbitals. orbitals.
The magnitude of J-T effectThe magnitude of J-T effect is related to the screening of the nuclear charge of is related to the screening of the nuclear charge of
the M-ion by the d electrons in the directions of the ligands. the M-ion by the d electrons in the directions of the ligands. It is smaller in Oh It is smaller in Oh
complexes with ground state configurations tcomplexes with ground state configurations t2g2g11, t, t2g2g
22, t, t2g2g44 and t and t2g2g
55 than in ground than in ground
state configurations tstate configurations t2g2g6 6 eegg
11, t, t2g2g6 6 eegg
33, t, t2g2g3 3 eegg
11, etc, etc. .
In fact, it has not been possible to detect J-T effect in Oh complexes of M-ions In fact, it has not been possible to detect J-T effect in Oh complexes of M-ions
with ground state configuration twith ground state configuration t2g2g11, t, t2g2g
22, t, t2g2g44 and t and t2g2g
55 (except from indirect (except from indirect
spectroscopic evidence) because the magnitude of the effect is comparatively spectroscopic evidence) because the magnitude of the effect is comparatively
very small and these complexes get little bid more stabilized due to J-T effect. very small and these complexes get little bid more stabilized due to J-T effect.
Cu+2 (d9) ion: How the d orbital energy levels change in Cu+2 (d9) ion when the Cu+2 (d9) ion: How the d orbital energy levels change in Cu+2 (d9) ion when the
regular Oh distorts? Let us consider the d-orbital energy levels change in the regular Oh distorts? Let us consider the d-orbital energy levels change in the
Cu+2 ion (d9 ion) when there occurs a small distortion of the type in which the Cu+2 ion (d9 ion) when there occurs a small distortion of the type in which the
regular Oh becomes stretched along z-axis. regular Oh becomes stretched along z-axis.
Here the splitting of the more stable Oh distortion corresponding to config. 1 Here the splitting of the more stable Oh distortion corresponding to config. 1
has been considered, has been considered, δδ11 and and δδ22, which represent the splitting of the e, which represent the splitting of the egg33 and t and t2g2g
66--
levels respectively, both are much smaller than Δlevels respectively, both are much smaller than Δoo and and δδ22 is much smaller than is much smaller than
δδ11..
ΔΔoo >>> >>> δδ11> > δδ22
TheThe two etwo egg-orbitals separate where d-orbitals separate where dxx22-y-y2 goes up as much as the other d2 goes up as much as the other dzz2 goes 2 goes
down; the tdown; the t2g2g orbitals separate so that doubly degenerate pair (d orbitals separate so that doubly degenerate pair (dyzyz and d and dxzxz) goes ) goes
down only half as far as the single orbital ddown only half as far as the single orbital dxyxy goes up. goes up.
Hence, net energy change for tHence, net energy change for t2g2g-electrons are zero, since four electrons (namely -electrons are zero, since four electrons (namely
ddzz2) goes down, the t2) goes down, the t2g2g orbitals separate so that doubly degenerate pair (d orbitals separate so that doubly degenerate pair (dyzyz and and
ddxzxz) are stabilized by 4 x (-1/3 ) are stabilized by 4 x (-1/3 δδ22) = - 4/3 ) = - 4/3 δδ22, while two electrons (d, while two electrons (dxyxy electrons) are electrons) are
destabilized by 2 x (+2/3 destabilized by 2 x (+2/3 δδ22) = +4/3 ) = +4/3 δδ22. Thus in the splitting of t. Thus in the splitting of t2g2g-levels:-levels:
Net energy gain = energy gain + energy loss = + 4/3 Net energy gain = energy gain + energy loss = + 4/3 δδ22 + (-4/3 + (-4/3 δδ22) =0) =0
The decrease in distortions energy due to J-T effect may be calculated as The decrease in distortions energy due to J-T effect may be calculated as
follows:follows:
1. Elongation along Z axis (1. Elongation along Z axis (possibilitypossibility): Energy of the complex corresponding to ): Energy of the complex corresponding to
elongation, Eelongation, E11= 4 x (- 4 Dq - ∂= 4 x (- 4 Dq - ∂22/3) + 2 x (- 4 Dq + 2 ∂/3) + 2 x (- 4 Dq + 2 ∂22/3) + 2 x (6 Dq - ∂/3) + 2 x (6 Dq - ∂11/2) + 1 x (6 Dq /2) + 1 x (6 Dq
+ ∂+ ∂11/2) = - 6 Dq - ∂/2) = - 6 Dq - ∂11/2/2
Energy of the complex without distortion is given byEnergy of the complex without distortion is given by
EE22 = 6 x (-4 Dq) + 3 x 6 Dq = - 6 Dq = 6 x (-4 Dq) + 3 x 6 Dq = - 6 Dq
Decrease in energy due to J-T effect distortion is given by Decrease in energy due to J-T effect distortion is given by
EE22 – E – E11 = - 6 Dq – (- 6 Dq - ∂ = - 6 Dq – (- 6 Dq - ∂11/2) = ∂/2) = ∂11/2. /2.
Thus increase in stability due to J-T distortion by ∂Thus increase in stability due to J-T distortion by ∂11/2./2.
2. Compression along Z axis (2. Compression along Z axis (possibilitypossibility): E): E33= 2 x (- 4 Dq – 2 ∂= 2 x (- 4 Dq – 2 ∂22/3) + 4 x (- 4 Dq + /3) + 4 x (- 4 Dq +
∂∂22/3) + 2 x (6 Dq - ∂/3) + 2 x (6 Dq - ∂11/2) + 1 x (6 Dq + ∂/2) + 1 x (6 Dq + ∂11/2) = - 6 Dq - ∂/2) = - 6 Dq - ∂11/2/2
Energy of the complex without distortion is given byEnergy of the complex without distortion is given by
EE22 = 6 x (-4 Dq) + 3 x 6 Dq = - 6 Dq = 6 x (-4 Dq) + 3 x 6 Dq = - 6 Dq
Decrease in energy due to J-T effect distortion is given by Decrease in energy due to J-T effect distortion is given by
EE22 – E – E33 = - 6 Dq – (- 6 Dq - ∂ = - 6 Dq – (- 6 Dq - ∂11/2) = ∂/2) = ∂11/2. /2.
Thus increase in stability due to J-T distortion by ∂Thus increase in stability due to J-T distortion by ∂11/2. /2.
To sum up: 1. J-T distortions occur in Oh complexes in which metal ions have To sum up: 1. J-T distortions occur in Oh complexes in which metal ions have
the ground state config. t2g3 eg1, t2g4 eg1, t2g4 eg2, t2g5 eg2, t2g6 eg1 and the ground state config. t2g3 eg1, t2g4 eg1, t2g4 eg2, t2g5 eg2, t2g6 eg1 and
t2g6 eg3, t2g1, t2g2, t2g4 and t2g5.t2g6 eg3, t2g1, t2g2, t2g4 and t2g5.
2. J-T distortions are automatic not imposed phenomenon.2. J-T distortions are automatic not imposed phenomenon.
3. J-T effect is operative with anionic ligands and neutral too.3. J-T effect is operative with anionic ligands and neutral too.
4. J-T effect shown by t2g orbitals is much weaker than that of eg orbitals.4. J-T effect shown by t2g orbitals is much weaker than that of eg orbitals.
Jahn-Teller effect on Electronic spectra of ComplexesJahn-Teller effect on Electronic spectra of Complexes
Electronic absorption bands in the spectra of coordination complexes are Electronic absorption bands in the spectra of coordination complexes are
associated with d-d transitions (excitation of electrons from tassociated with d-d transitions (excitation of electrons from t2g2g to e to egg orbitals). orbitals).
Thus, the frequency of electromagnetic radiation absorbed should matches with Thus, the frequency of electromagnetic radiation absorbed should matches with
the frequency of the two d orbitals involved in the electronic transition. For the frequency of the two d orbitals involved in the electronic transition. For
example, [Ti(Hexample, [Ti(H22O)O)66]]3+3+- - Oh complexOh complex, (d, (d11 system). In ground state, electron system). In ground state, electron
occupies toccupies t2g2g orbitals and e orbitals and egg remains vacant. This complex absorbs in the visible remains vacant. This complex absorbs in the visible
region around region around λλ= = 5000 Ǻ. The energy absorbed excites the solitary electron to 5000 Ǻ. The energy absorbed excites the solitary electron to
one of the eone of the egg orbitals. The t orbitals. The t2g2g11 → e → egg
11 electron transition should give rise to a electron transition should give rise to a
single symmetrical absorption band in the spectrum of [Ti(Hsingle symmetrical absorption band in the spectrum of [Ti(H22O)O)66]]+3+3. . But is not But is not
soso. .
The observed absorption bond is unsymmetrical, which result of overlapping of The observed absorption bond is unsymmetrical, which result of overlapping of
more than one absorption bands. more than one absorption bands. This can be accounted for on the basis of J-T This can be accounted for on the basis of J-T
effect as followseffect as follows::
% A
BS
OR
PT
ION
1000 1500 2000 2500 3000
(cm-1)
(Unsymmetrical absorption band observed in the spectrum of [Ti(H(Unsymmetrical absorption band observed in the spectrum of [Ti(H22O)O)66]]+3+3).).
SupposeSuppose, the ground state electron is in d, the ground state electron is in dxyxy orbital ( orbital (slightly stable complexslightly stable complex). ).
ReasonReason: complex distorted to tetragonal geometry in which the Ti-OH: complex distorted to tetragonal geometry in which the Ti-OH22 bonds bonds
along z axis are shorter than those along x and y axes (J-T effect). The energy of along z axis are shorter than those along x and y axes (J-T effect). The energy of
ddxyxy orbital gets lower than d orbital gets lower than dxzxz and d and dyzyz orbitals and energy gap δ orbitals and energy gap δ22 between d between dxyxy and and
(d(dxzxz, d, dyzyz) is much smaller than the average width of an electronic absorption ) is much smaller than the average width of an electronic absorption
band. band. Gap δGap δ22 can neither account for absorption of electromagnetic radiation can neither account for absorption of electromagnetic radiation
(visible region) nor can account for the asymmetry in the electronic absorption (visible region) nor can account for the asymmetry in the electronic absorption
band of [Ti(Hband of [Ti(H22O)O)66]]+3+3. .
Thus, dThus, dxyxy electron on excitation may occupy either (i) d electron on excitation may occupy either (i) dxx2-2-yy2 orbital or (ii) d2 orbital or (ii) dzz2 2
orbital. If the electron occupies dorbital. If the electron occupies dxx2-2-yy2 orbital in the excited state, charge 2 orbital in the excited state, charge
polarized more in the xy plane and if in dpolarized more in the xy plane and if in dzz2 orbital it will be along the z direction. 2 orbital it will be along the z direction.
This results in two electronic sets in the excited state having different energy. This results in two electronic sets in the excited state having different energy.
Lowering of the symmetry of [Ti(HLowering of the symmetry of [Ti(H22O)O)66]]+3+3 from Oh to tetragonal due to J-T effect, from Oh to tetragonal due to J-T effect,
splits the excited state and makes possible two electronic transitions or splits the excited state and makes possible two electronic transitions or
electronic jumps, viz. delectronic jumps, viz. dxyxy11 → d → dxx2-2-yy2211 and d and dxyxy
11 → d → dzz2211, which should give rise to , which should give rise to
two absorption bands. two absorption bands. Since the energy difference δSince the energy difference δ22 between d between dxyxy and d and dxzxz, d, dyzyz is is
very small, the population difference between dvery small, the population difference between dxyxy and d and dxzxz, d, dyzyz levels is also very levels is also very
small.small. Therefore, the ground state of [Ti(H Therefore, the ground state of [Ti(H22O)O)66]]+3+3 is often written as tis often written as t2g2g11 and and
transition as ttransition as t2g2g11 → d → dxx2-2-yy2211 and t and t2g2g
11 → d → dzz2211. .
E
dz2
dx2-y
2
dxz, dyz
dxy
E1 E2
(Splitting of the excited state energy levels due to J-T (Splitting of the excited state energy levels due to J-T effect in [Ti(Heffect in [Ti(H22O)O)66]]+3+3 in the distorted geometry making in the distorted geometry making
possible two electronic transitions).possible two electronic transitions).
ConverselyConversely, a considerable fraction of the complex ions at any moment present , a considerable fraction of the complex ions at any moment present
containing the other possible tetragonally distorted geometry in which Ti-OHcontaining the other possible tetragonally distorted geometry in which Ti-OH22
bonds along the z direction are elongated compared to other such bonds due to bonds along the z direction are elongated compared to other such bonds due to
the following reason: the following reason:
The distorted geometry of Ti(HThe distorted geometry of Ti(H22O)O)66]]+3+3 obtained by compression of Ti-OH obtained by compression of Ti-OH22 bonds bonds
along z axis is stable by δalong z axis is stable by δ22/3 over its distorted geometry obtained by elongation /3 over its distorted geometry obtained by elongation
of Ti-OHof Ti-OH22 bonds along z axis. But the difference δ bonds along z axis. But the difference δ22/3 in the energies of the two /3 in the energies of the two
distorted geometries of Ti(Hdistorted geometries of Ti(H22O)O)66]]+3+3 is very small and the thermal energy available is very small and the thermal energy available
at rt to these complex ions in the distorted geometry is sufficiently enough to at rt to these complex ions in the distorted geometry is sufficiently enough to
allow these ions to change their geometry.allow these ions to change their geometry.
Factors determine the geometry of the complex ionsFactors determine the geometry of the complex ions::
1. J-T effect tends to favor the distorted geometry for Ti(H1. J-T effect tends to favor the distorted geometry for Ti(H22O)O)66]]+3+3 in which d in which dxyxy is is
the lowest energy orbital.the lowest energy orbital.
2. The thermal energy available to the complex tends to equalize the population 2. The thermal energy available to the complex tends to equalize the population
of the complex ions in the two distorted geometries. Due to this, an equilibrium of the complex ions in the two distorted geometries. Due to this, an equilibrium
condition between the populations of the complex ions in the two distorted condition between the populations of the complex ions in the two distorted
geometries exists. This is known as geometries exists. This is known as dynamic J-T effectdynamic J-T effect. Thus for dynamic J-T . Thus for dynamic J-T
effect energy gap of teffect energy gap of t2g2g split up and e split up and egg split up should be small. split up should be small.
3. In Oh complex, the energy gap between t3. In Oh complex, the energy gap between t2g2g & e & egg is quite high. The thermal is quite high. The thermal
energy available at rt is not enough to influence the population of the molecules energy available at rt is not enough to influence the population of the molecules
in any distorted geometries. Hence, J-T effect will solely determine the stable in any distorted geometries. Hence, J-T effect will solely determine the stable
distorted geometry in which the complex will acquire. Thus, all the complexes distorted geometry in which the complex will acquire. Thus, all the complexes
will exist in only one stable distorted geometry is known as will exist in only one stable distorted geometry is known as static J-T effectstatic J-T effect. .
Here,Here,[Ti(H[Ti(H22O)O)66]]+3+3 exhibits dynamic J-T effect at rt exhibits dynamic J-T effect at rt (complex ions having both the (complex ions having both the
types of distorted geometries exist in equilibrium). The electron absorption types of distorted geometries exist in equilibrium). The electron absorption
spectra of [Ti(Hspectra of [Ti(H22O)O)66]]+3+3 will be an average of the electron absorption spectra of the will be an average of the electron absorption spectra of the
two distorted geometries.two distorted geometries.
Let us see what type of electron absorption spectra will be observed for the Let us see what type of electron absorption spectra will be observed for the
other distorted geometry in which Ti-OHother distorted geometry in which Ti-OH22 bonds along z axis are longer than bonds along z axis are longer than
bonds along x and y directions. bonds along x and y directions. The ground state for such a geometry will be The ground state for such a geometry will be
(d(dxzxz, d, dyzyz))11. The excited state d. The excited state dzz2211 will be of lower energy than the excited state will be of lower energy than the excited state
ddxx2-2-yy2211. Thus, we can observed two transitions; t. Thus, we can observed two transitions; t2g2g11 → d → dxx2-2-yy2211 and t and t2g2g
11 → d → dzz2211. .
Since ΔESince ΔE11 (Ed (Edxx2-2-yy2 - Ed2 - Edxzxz, d, dyzyz) is close to ΔE) is close to ΔE22 (Edz (Edz22 – Ed – Edxzxz, d, dyzyz) , the two ) , the two
transitions will again give rise to two overlapping bands (unsymmetrical band). transitions will again give rise to two overlapping bands (unsymmetrical band).
Thus, an unsymmetric absorption band will be observed whether the complex Thus, an unsymmetric absorption band will be observed whether the complex
ion [Ti(Hion [Ti(H22O)O)66]]+3+3 has the distorted geometries and an equilibrium exists between has the distorted geometries and an equilibrium exists between
them due to dynamic J-T effect. them due to dynamic J-T effect.
(Splitting of the excited state energy levels due to J-T effect (Splitting of the excited state energy levels due to J-T effect in [Ti(Hin [Ti(H22O)O)66]]+3+3 in the distorted geometry making possible two in the distorted geometry making possible two
electronic transitions).electronic transitions).
E
dz2
dx2
-y2
dxz, dyz
dxy
E1 E2
Consider, CuConsider, Cu+2+2:: vizviz. [Cu(H. [Cu(H22O)O)66]]+2+2. Mostly Oh complexes of Cu. Mostly Oh complexes of Cu+2+2 exhibit tetragonal exhibit tetragonal
distortion because of J-T effect in which Cu-L bonds along z axis are elongated distortion because of J-T effect in which Cu-L bonds along z axis are elongated
compared to the bonds in the xy plane. compared to the bonds in the xy plane.
t2g6
eg3
dxy
dxz, dyz
dz2
dx2-y2
3 12
(Possible electronic transition in tetragonal [Cu(H(Possible electronic transition in tetragonal [Cu(H22O)O)66]]+2+2). ).
Discuss the electronic absorption spectrum of this geometry only. Unlike TiDiscuss the electronic absorption spectrum of this geometry only. Unlike Ti+3+3
complexes, splitting of ecomplexes, splitting of egg orbitals in Oh Cu orbitals in Oh Cu+2+2 complexes gave the energy of the complexes gave the energy of the
ddzz2 orbital decreased close to the energies of d2 orbital decreased close to the energies of dxyxy & (d & (dxzxz, d, dyzyz) orbitals (J-T effect). ) orbitals (J-T effect).
Therefore, the energy required to excite an electron from any one of these Therefore, the energy required to excite an electron from any one of these
orbitals to the partially vacant dorbitals to the partially vacant dxx2-2-yy2 orbital is nearly of the same magnitude. 2 orbital is nearly of the same magnitude.
Thus, the following three electronic transitions are possible in electronic spectra Thus, the following three electronic transitions are possible in electronic spectra
of [Cu(Hof [Cu(H22O)O)66]]+2+2 : :
(i) Transition of an electron from d(i) Transition of an electron from dzz2 ---- d2 ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν11..
(ii) Transition of an electron from d(ii) Transition of an electron from dxyxy ---- d ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν22..
(iii) Transition of an electron from d(iii) Transition of an electron from dxzxz ---- d ---- dxx2-2-yy2 orbital by absorbing energy hν2 orbital by absorbing energy hν33..
The three transitions of close energy would give rise to three absorption bands at The three transitions of close energy would give rise to three absorption bands at
close frequencies νclose frequencies ν11, ν, ν22 and ν and ν33 which overlap to give a composite, unsymmetrical which overlap to give a composite, unsymmetrical
absorption band as shown:absorption band as shown:
% A
BS
OR
PT
ION
5000 10000 15000
(cm-1)
If no J-T distortion, the spectrum would have consisted of a single symmetric If no J-T distortion, the spectrum would have consisted of a single symmetric
band corresponding to the transition tband corresponding to the transition t2g2g66eegg
33 → t → t2g2g55eegg
44. The observed spectrum of . The observed spectrum of
the complex explained only on the basis of the J-T effect.the complex explained only on the basis of the J-T effect.
Limitations of CFTLimitations of CFT
(i) CFT considers only the metal ion d-orbitals and gives no consideration at all (i) CFT considers only the metal ion d-orbitals and gives no consideration at all
to other metal orbitals such as s-, p-orbitals and the ligand to other metal orbitals such as s-, p-orbitals and the ligand ππ-orbitals. Therefore, -orbitals. Therefore,
to explain all the properties of the complexes dependent on the to explain all the properties of the complexes dependent on the ππ-ligand orbitals -ligand orbitals
will be outside the scope of CFT. This does not consider the formation of will be outside the scope of CFT. This does not consider the formation of ππ--
bonding in complexes.bonding in complexes.
(ii) CFT is unable to account satisfactorily for the relative strengths of ligands, (ii) CFT is unable to account satisfactorily for the relative strengths of ligands,
e.g. it gives no explanation as to why He.g. it gives no explanation as to why H22O appears in the spectrochemical series O appears in the spectrochemical series
as a stronger ligand than OHas a stronger ligand than OH--..
(iii) According to CFT, the bond between the metal and ligands are purely ionic. (iii) According to CFT, the bond between the metal and ligands are purely ionic.
It gives no account of the partly covalent nature of the metal-ligand bonds. Thus It gives no account of the partly covalent nature of the metal-ligand bonds. Thus
the effects directly dependent on covalently cannot be explained by CFT. the effects directly dependent on covalently cannot be explained by CFT.
Ligand-Field TheoryLigand-Field Theory
The VBT and CFT explain some aspects of all of the properties of transition-The VBT and CFT explain some aspects of all of the properties of transition-
metal complexes. So, metal complexes. So, ligand-field theoryligand-field theory developeddeveloped based on MOT. based on MOT.
The ligand-field model for an octahedral transition-metal complex such as The ligand-field model for an octahedral transition-metal complex such as
[Co(NH[Co(NH33))66]]3+3+ ion assumes that the 3 ion assumes that the 3dd, 4, 4ss, and 4, and 4pp orbitals on the metal overlap orbitals on the metal overlap
with one orbital on each of the six ligands to form a total of 15 molecular with one orbital on each of the six ligands to form a total of 15 molecular
orbitals. orbitals.
Six-Six-bonding molecular orbitalsbonding molecular orbitals, whose energies are much lower than those of , whose energies are much lower than those of
the original atomic orbitals. Six-the original atomic orbitals. Six-antibonding molecular orbitalsantibonding molecular orbitals, whose , whose
energies are higher than those of the original atomic orbitals. Three-energies are higher than those of the original atomic orbitals. Three-
nonbonding molecular orbitalsnonbonding molecular orbitals, because they have essentially the same , because they have essentially the same
energy as the 3energy as the 3dd atomic orbitals on the metal. atomic orbitals on the metal.
Ligand-field theory enables the 3Ligand-field theory enables the 3dd, 4, 4ss, and 4, and 4pp orbitals on the metal to overlap orbitals on the metal to overlap
with orbitals on the ligand to form the octahedral covalent bond skeleton that with orbitals on the ligand to form the octahedral covalent bond skeleton that
holds this complex together. At the same time, this model generates a set of holds this complex together. At the same time, this model generates a set of
five orbitals in the center of the diagram that are split into five orbitals in the center of the diagram that are split into tt22gg and and eegg subshells, subshells,
as predicted by the crystal-field theory. As a result, we don't have to worry as predicted by the crystal-field theory. As a result, we don't have to worry
about "inner-shell" versus "outer-shell" metal complexes. In effect, we can use about "inner-shell" versus "outer-shell" metal complexes. In effect, we can use
the 3the 3dd orbitals in two different ways. We can use them to form the covalent orbitals in two different ways. We can use them to form the covalent
bond skeleton and then use them again to form the orbitals that hold the bond skeleton and then use them again to form the orbitals that hold the
electrons that were originally in the 3electrons that were originally in the 3dd orbitals of the transition metal. orbitals of the transition metal.
If an object absorbs all colors but one, we see the color it does not absorb. The yellow strip in the following figure absorbs red, orange, green, blue, indigo and violet light. It reflects yellow light and we see it as yellow.
An object is seen as black if it absorbs all colors of white light. A white object reflects all colors of white light equally.
The yellow strip in the following figure looks yellow because it absorbs indigo light from white light. Indigo is the complementary color of yellow.
A solution containing the complex ion, [Cu(NH3)4]2+, is blue because the complex absorbs red and orange light, the complementary colors of blue and blue-green.
When the d-level is not completely filled, it is possible to promote and electron from a lower energy d-orbital to a higher energy d-orbital by absorption of a photon of electromagnetic radiation having an appropriate energy. Electromagnetic radiations in the visible region of the spectrum often possess the appropriate energy for such transitions.
Although visible light appears "white", it is made up of a series of colors. White light consists of three primary colors (red, yellow and blue). These primary colors can be mixed to make three secondary colors (orange, green and violet).
Red + Yellow makes Orange Yellow + Blue makes Green Blue + Red makes Violet
Seeing ColorThe sensors in our eyes detect only those wavelengths in the visible portion of the electromagnetic spectrum.
Grass and leaves appear green because chlorophyll absorbs wavelengths in the red and blue portion of the visible spectrum. The wavelengths in between (green) are transmitted.
Transition Metal ComplexesWhen light passes through a solution containing transition metal complexes, we see those wavelengths of light that are transmitted. The solutions of most octahedral Cu (II) complexes are blue. The visible spectrum for an aqueous solution of Cu (II), [Cu(H2O6]2+, shows that the absorption band spans the red-orange-yellow portion of the spectrum and green, blue and violet are transmitted.
The absorption band corresponds to the energy required to excite an electron from the t2g level to the eg level.
Recall, the energy possessed by a light wave is inversely proportional to its wavelength. The Cu(II) solution transmits relatively high energy waves and absorbs the low energy wavelengths. This indicates that the band gap between the two levels is relatively small for this ion in aqueous solution.
d-Orbital SplittingThe magnitude of the splitting of the d-orbitals in a transition metal complex depends on three things: 1. the geometry of the complex 2. the oxidation state of the metal 3. the nature of the ligands The Nature of the LigandsSome ligands only produce a small energy separation among the d-orbitals while others cause a wider band gap. Ligands that cause a small separation are called weak field ligands, and those that cause a large separation are called strong field ligands. The ordering of their splitting ability is called the spectrochemical series.
A comparison of the visible absorption maxima for a number of cobalt (III) complexes shows the effects of ligands on the d-orbital band gap.
The magnetic properties of materials were recognized by the ancient Greeks, Romans, and Chinese, who were familiar with lodestone, an iron oxide mineral that attracts iron objects. Although the attractive or repulsive forces that act between magnetic materials are manifestations of magnetism familiar to everybody, the origin of magnetism lies in the atomic structure of matter. Despite the fact that magnetism can be explained only by the quantum theory developed at the beginning of the twentieth century, qualitative predictions of magnetic properties can be made within the context of classical physics. Magnetic forces originate in the motion of charged particles, such as electrons. The electrons "spin" around their axis and move in orbits around the nucleus of the atom to which they belong. Both motions generate tiny electric currents in closed loops that in turn create magnetic dipole fields, just as the current in a coil does. When placed in a magnetic field, the tiny magnetic dipole fields tend to align with the external field.According to their behavior in inhomogeneous magnetic fields, materials can be classified into three main categories: diamagnetic, paramagnetic, and ferromagnetic. Paramagnetic materials are attracted into a magnetic field. The main cause of this effect is the presence in the material of atoms that have a net magnetic moment composed of electron spin and orbital contributions.
Magnetism
Iron filings in a circular pattern around a magnet, indicative of the field of force of the magnet. When placed in a magnetic field, the magnetic moments of the atoms, which are otherwise randomly oriented, tend to align with the field and thus enhance the field. Paramagnetism is temperature dependent because increased thermal motion at higher temperatures impedes the alignment of the magnetic moments with the field. Diamagnetic materials are slightly repelled by a magnetic field. This effect occurs for materials that contain atoms in which the spin and orbital contributions to the magnetic moment cancel out. In this case, the interaction between the material and a magnetic field is caused by the occurrence of currents induced by the magnetic field in the atoms. The dipole fields corresponding to these currents are directed opposite to the applied magnetic field and cause expulsion of the material from the field.
Ferromagnetic materials contain atoms that have magnetic moments that are aligned even in the absence of an applied magnetic field because of mutual interactions, creating a sizable net magnetic moment for domains of the material. The magnetic moments of domains can be randomly oriented unless a magnetic field is applied to the material.Iron, cobalt, nickel, and their alloys are examples of ferromagnetic materials. These three elements are transition metals, and their atoms or ions have unpaired electrons in d orbitals. Rare-earth ions also have unpaired electrons situated in f orbitals. A detailed investigation of the properties of molecules that contain such metal ions in a magnetic field can provide significant information about how their electrons are distributed in orbitals. Typically, d orbitals of isolated atoms are degenerate (Figure 1a). This situation changes when the metal ions are part of molecules in which they experience a nonspherically symmetric environment. Figures 1b and 1c show the splitting of d orbitals for a transition metal ion that has six unpaired electrons and is situated in an environment of six atoms in an octahedral arrangement. Depending on the size of the splitting (the lighter shading in Figure 1) and the interelectron repulsion, the metal ion may have four unpaired electrons (Figure 1b) or no unpaired electrons (Figure 1c).
This difference in electron distribution leads to significant differences in the magnetic properties of the molecules that contain such ions, with the former being paramagnetic and the latter being diamagnetic. When there are multiple metal sites in a molecule, the spins at different metal ions can be either ferro-(parallel) or antiferro-magnetically (antiparallel) aligned to each other. Clever use of the magnetic properties for metal ions and of the interactions between spins manifested in molecular systems enables scientists to design and synthesize molecular systems with interesting properties, such as molecular magnets.
Magnetic materials are widely used for building technological devices and scientific tools. Classical examples are electromagnets that are used in motors, clutches, and breaking systems. The electromagnet makes use of an iron core situated in a solenoid through which electric current is passed. This current creates a magnetic field at the center of the solenoid that orients the magnetic moments in the domains of the iron core, which in turn results in a significant enhancement of the magnetic field at the core of the solenoid. Electromagnets can also be used to record information on magnetic tape, which has a ferromagnetic surface.Finally, although atomic nuclei have significantly smaller magnetic moments than electrons, the study of their interaction with magnetic fields has many important applications. They enable the scientists in the biological and medical fields to elucidate the structure of biologically relevant molecules such as proteins and to diagnose diseases using magnetic resonance imaging.
Magnetism in Transition Metal Complexes
Electronic Absorption Spectra of Metal Complexes explains in detail the origin of color in complexes of the transition metals. We learn that 6 sigma bonding ligands disposed about a metal ion at the vertices of a regular octahedron cause a splitting of the d orbitals of the metal into two sets, one triply degenerate set, n, of non-bonding orbitals (dxy,xz,yz), and one doubly degenerate set, s*, of antibonding orbitals (dz2,x2-y2). If these orbitals are partially filled with electrons originally belonging to the metal, and if the ligand field splitting, Do, between the two d orbital sets is such that visible light can cause the transition of an electron from the lower to the upper set, then the complex will be colored. We will now see that the very interesting and variable magnetic behavior of transition metals in complexes has its origin in the same basic structural features--split, partially filled d orbitals. With a couple of additional concepts, we can develop a satisfactory explanation of this magnetic behavior on the basis of the same theory that we used in discussing electronic spectra and color. We begin with an examination of the various types of magnetic behavior displayed by matter.
When a piece of matter is placed in a magnetic field, H (italic signifies a vector quantity), the matter becomes magnetically polarized; that is, a magnetic field is set up in the matter as a result of its presence within the external field, and it has been found experimentally that the strength of the internal field is proportional to the strength of the applied (external) field. We express this mathematically by equation (1) where M is the magnetic polarization of the matter and c is a(K-1): M = cH proportionality constant called the volume magnetic susceptibility. It has been further demonstrated by experiment that c may be either negative or positive. Materials for which c is small and less than 0 are called diamagnetic materials. Such behavior is always found for materials that contain no unpaired electrons. Most (but not all) organic compounds qualify as diamagnetic materials, since all their electrons are paired. Diamagnetic behavior is due to small fields induced in the sample by the applied field that are absent if no external field is present. All materials, regardless of any additional magnetic behavior they may display, exhibit diamagnetism. Materials for which c is greater than 0 are found to contain one or more unpaired electrons. If 0 < c <1 (the usual situation), the material is said to be paramagnetic. If on the other hand c > 1, the material is termed ferromagnetic.
Notice that c being greater than 1 means that the magnetic field set up in the sample of matter is greater in magnitude than the applied field! This behavior is exhibited by metallic iron (hence the word ferromagnetic), nickel, and some other metals. Ferromagnetism, however interesting, will not concern us any more at present. We are interested now in the phenomenon of paramagnetism, which arises whenever a molecule contains unpaired electrons. Oxygen is paramagnetic because it contains two unpaired electrons. NO2 is paramagnetic since it contains an odd number of electrons. (Any substance containing an odd number of electrons must be paramagnetic. Nature usually avoids this situation in compounds of the representative elements, but makes up for it in transition and inner transition elements, where there can be as many as 7 unpaired electrons per molecule.) Organic free radicals are paramagnetic due to the presence of an odd electron (remember the methyl radical, CH3, in free radical halogenation?). As it happens, complexes of most transition metals are paramagnetic as a result of the presence of unpaired electrons in the split d orbitals. We have already encountered an example of this in the complex Cr(NH3)63+, which contains 3 unpaired electrons, as discussed in the Appendix J.
Consider another example, this time involving FeIII, whose electronic configuration is [Ar]3d5 in the absence of ligands. We would expect the free gaseous ion to contain 5 unpaired electrons, as indeed it does--the electrons are distributed in the five degenerate d orbitals, according to the Pauli Exclusion Principle and Hund's Rules, as shown below:
Hund's Rules - electrons filling a set of degenerate orbitals will fill them so as to maintain their spins parallel for as long as possible. Similarly, the complex Fe(H2O)63+, present in strongly acid aqueous solution, contains 5 unpaired electrons. However, the very stable complex Fe(CN)63-, named hexacyanoferrate(-3), contains only one unpaired electron, even though it also contains Fe(III). We deduce from this that the nature of the ligands must in some way influence the pairing of electrons. But how? Let's call on what we have learned about the splitting of the d orbitals by the ligands. Figure 2 shows the situation in terms of an energy level diagram.
In the complex, 5 electrons must distribute themselves among the d orbitals,which as a result of the presence of 6 ligands are no longer all degenerate. There is no ambiguity about where the first three electrons will go--one electron will enter each of the three degenerate orbitals in the n set. The fourth electron, however, is faced with a choice: it can either pair up with one of the electrons in a lower orbital, requiring the expenditure of an amount of energy, P, called the pairing energy; or it can occupy one of the orbitals in the upper doubly degenerate set at the expense of the energy, Do , since the upper set, s*, is less stable by than the lower set, n. The electron will take the option which requires the expenditure of the least energy. If Do < P, it will occupy one of the s* orbitals. If Do > P, the electron will pair up in n. The fifth electron will be faced with the same choice, and will make the same decision. Two situations are therefore possible in octahedral complexes of FeIII:
In one case, five unpaired electrons are present. This is called the high spin (HS) case and must correspond to the situation in Fe(H2O)63+. In the second case, called the low spin (LS) case, only one unpaired electron is present--the maximum possible amount of electron pairing has occurred. This is the situation which in Fe(CN)63-. We conclude, then, that when H2O is the ligand, Do < P and that when CN- is the ligand, Do > P (P remains essentially the same for a given metal ion, regardless of ligand, and can be obtained from spectroscopic measurements of the free metal ion, in this case FeIII.) We call water a relatively "weak-field ligand" and cyanide a "strong-field ligand."
Any ion having as few as 4 or as many as 7 d electrons can exhibit either high- or low-spin behavior in an octahedral complex. You should work out the numbers of unpaired electrons expected in octahedral complexes of metal ions with configurations d1 to d9 to convince yourself of this. The theory that we have developed above explains beautifully most aspects of the magnetic behavior of transition metal complexes, the exceptions being the more subtle aspects that we will avoid for the time being. Let's turn now to the relationship between the number of unpaired electrons possessed by a molecule and its behavior in a magnetic field.There are two sources of magnetic behavior in matter: 1) orbital motion of electrons; 2) spin of electrons. We will deal with spin motion only now, since it is the most important source of magnetic behavior in complexes of transition metals. Pictured below is an electron spinning (rotating) about its own axis.
This motion is analogous to the rotation of the earth about its axis. Since the electron is a charged particle, this rotational motion constitutes an electric current. To see this more clearly, mentally replace the electron with a loop of wire in which a current is flowing:
Recall from your study of electricity and magnetism in physics, that a flow of current always generates a magnetic field. When the current is flowing in a loop, the direction of the magnetic field is perpendicular to the plane of the loop. Thus
The spin of the electron about its axis, since it is an electric current, similarly generates a magnetic field parallel to the axis of rotation. This magnetic field is called the spin magnetic moment of the electron, and is given the symbol mS. The arrow over the symbol indicates that the magnetic moment is a vector quantity. Now rotational motion of any kind always generates angular momentum, which is also a vector parallel to the axis of rotation. The magnetic moment and the angular momentum are therefore colinear vectors. Furthermore, their magnitudes are directly proportional. This is indicated in the following equation
(K-2): m = gL where L = angular momentum and g is the proportionality constant relating magnetic moment and angular momentum. When a paramagnetic sample is placed in an external magnetic field, the individual spin magnetic moments of the unpaired electrons all line up with the field, just as iron filings line up along the magnetic lines of flux generated by a bar magnet. Their vector sum is the magnetic polarization defined by equation (1). Thus we see that magnetic polarization, which is a macroscopic property, is related to individual spin magnetic moments, which are microscopic properties of the system.It can be shown that the magnetic moment of an individual ion in a sample is related to the number of unpaired electrons on the ion by the following equation.(K-3): mS = [n(n + 2)]1/2 Bohr magnetons The Bohr magneton is a convenient unit for expressing magnetic moments, and is defined in terms of fundamental constants as 1 BM = eh/4pmc. Here e = the charge on the electron, h = Planck's constant, m = the mass (K-4): mS = 2.84(cMT)1/2