geometric isomerism - midwestern state...
TRANSCRIPT
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Geometric Isomerism
• cis => 2 adjacent ligands• trans => 2 ligands across the center of
coordination sphere
[PtCl2(NH3)2]
Geometric Isomerism• fac (facial) => three identical ligands
occupying the corners of a common triangular surface
• mer (meridional) => three identical ligands occupying three consecutive corners of a square plane
Cl
Cl
Cl
Cl
CL
CL
fac mer
Enantiomers of Oh complexesView down 3-fold axisChelating ligands define left or right handHelix rotation
Lower case letters are used for mirror imageStructures
δ λ
δ λ
D4h has inversion:This causes loss ofchirality
d-orbitals
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a1g Molecular Orbitals
t1g Molecular Orbitalseg Molecular Orbitals
Octahedral Field Splitting Pattern Splitting ofd-Orbitals in Oh Field
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Reducing axial ligand repulsions splitsThe degenerate eg and t2g orbitals
Jahn-Teller distortion
Octahedral MO
Diagram
Electronic Spectrum of [Ti(H2O)6]+3
t12g e0
g is ground state
1300 243 oKJcmmol
− = ≈ ∆
1max 20,300 243 o
KJcmmol
λ −= = ≈ ∆
Spectral Effects of Ligands and Metals
• Different binding atoms of ligands exert different repulsions on the metal orbitals
• Different oxidation states of the metal-increases with increasing oxidation number
• There is no quantified measure of energy changes as a result of these factors from crystal field theory
o∆
Spectral characteristics of octahedral Co complexes
Beer-Lambert Law: logo
I A bcI
ε= =
Io = light entering the substanceI = attenuated light exiting the substanceA = absorbanceb = path length, cmc = molar concentration
= transition probability (extinction coefficient)ε
o∆ Determines the color of complexes: t2g – eg
The energy of the wavelength corresponding tois
o∆
E hν=
4
Variation of ∆o
Metal ions of equal oxidation stateare placed in a spectrochemicalSeries:Mn(II)<Ni(II)<Co(II) <Fe(III)<Cr(III)<Co(III)<Ru(III)<Mo(III)<Rh(III)<Pd(III)<Ir(III)<Pt(IV)
Weak vs. Strong Field Splitting
Crystal Field Splitting Diagrams
Low spin High spin
Tetrahedral ML4
Tetrahedral ligandsImpinge upon the dOrbitals to a smallerDegree than Oh geometry
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Relationship of ∆t and ∆o
Relationship of Octahedral vs Square Planar Geometry
π-Orbitals in L-M-LOctahedral MO Diagram, with π-Bonding
Octahedral MO Diagram, with π-Bonding Pi bonding explains field strength of ligands
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Pi interactions are important in organo-metallic reactions
Pi backbonding: metalActs as Lewis base andProvides electron densityTo empty pi* orbitals ofligand
Microstate Term SymbolsLigand Field Theory
Mulliken symbols fromIrreducible tables
Donation ofe- density
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Atomic termSymbolLS coupling
Symmetry label hasSame multiplicity asParent term symbol.T2g and E2g derive fromD term symbol: assymetricDegenerate population
Irreducible has same symmetryAs term symbol: excited statesWith same multiplicity are More probable
F: A2g + T1g + T2gP: T1g
t2g2
t2g1eg
1
e22
LS coupling overcomeBy ligand strength
B: Racah parameter-amount of repulsion between terms of samemultiplicity
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Forbidden transitions are very weak (of low probability)
Transition metal complexes have color as a result of vibronicdistortions away from symmetry
High and low spin complexes have unique propertiesincluding magnetic field interactions, ligand association, and Spectral differences.
(a) paramagnetism,(b) ferromagnetism, (c) antiferromagnetism,
and (d) ferrimagnetism
Orientation of d-metal complexes in solids produces bulk magneticProperties. Magentic field of complex is measured in Bohr magnetons:
12( ( 1)n nµ = +
LFSE
Pairing energyvs LFSE
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At room temperature, the observed value of µeff for [Cr(en)3]Br2 is 4.75 µB. Is the complex high- or low-spin?
[Cr(en)3]+2 has Cr+2, a d4 case_ _
_ eg*
……………………...……………………
t2ghigh spin
low spin 4 unpaired e-
2 unpaired e-
µ = (n(n + 2))1/2
n = 2 for low spin; µ = (8)1/2 = 2.83n = 4 for high spin; µ = (24)1/2 = 4.90
Ligand-field Theory
Octahedral Complexes_ _eg *
_ _ _ _ __ _ _ _ _
_ _ _t2g energy
free ion Hypothetical ion octahedralin a spherically field
symmetric field
Ligand-field Stabilization Energy
LFSE = x(-4Dq) + y(+6Dq)where
x = number of electrons in lower levelsy = number of electrons in upper levels
Ligand-field Stabilization Energy
Octahedral complexes_ _ _ _ _ _ eg
* ___ ___
+6Dq………………………………………………………………. 0 10Dq
-4DqX _ _ X X _ X X X t2g ___ ___
d1 d2 d3
-4Dq -8Dq -12Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
Ligand-field Stabilization Energy
Octahedral complexesd4 _ _
X _ eg*
………………………………………………
X X X t2g
XX X X weak field
strong field
-16Dq -6Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
Ligand-field Stabilization Energy
Octahedral complexesd5 _ _
X X eg*
………………………………………………
X X X t2gXX XX X weak field
strong field
-20Dq 0Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
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Ligand-field Stabilization Energy
Octahedral complexesd6 _ _
X X eg*
………………………………………………
XX X X t2g
XX XX XX weak field
strong field
-24Dq -4Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
Ligand-field Stabilization Energy
Octahedral complexesd7 X _
X X eg*
………………………………………………
XX XX X t2g
XX XX XX weak field
strong field
-18Dq -8Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
Ligand-field Stabilization Energy
Octahedral complexesX X XX X XX XX eg
* ___ ___
+6Dq………………………………………………………………. 0 10Dq
-4DqXX XX XX XX XX XX XX XX XX t2g ___ ___
d8 d9 d10
-12Dq -6Dq 0Dq LFSE
LFSE = x(-4Dq) + y(+6Dq)
Spectrochemical Series
Ligand Field StrengthCN- > phen ~ NO2
- > en > NH3 ~ py > H2O > C2O4
-2 > OH- > F- > S-2 > Cl- > Br- > I-