UNIT 5: COORDINATION CHEMISTRY AND TRANSITION ELEMENTSTransition Metal *CHY4006 GRH

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CONTENTCoordination Numbers and GeometriesTypes of LigandsIsomerismNomenclatureStability of Coordination CompoundsReactivity and Reaction Mechanisms of Coordination CompoundsLigand Field Theory and Ligand Field Splittings and SpectraCrystal Field ApproachMagnetic Properties of Transition Metal CompoundsChemistry of the first row transition elements

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Coordination CompoundsNature of coordination compounds is determined by both the oxidation number of the central ion and its coordination number.The coordination number is (mostly) constant for a metal with a given oxidation number.First coordination sphere: central metal ion and its ligandsFirst Coordination SphereCoordination number is often 2 x oxidation number (many exceptions)Mostly predictable geometry:Coordination number = 2, linearCoordination number = 4, square planar or tetrahedralCoordination number = 6, octahedralCHY4006 GRH*

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Coordination Compound

Consist of a complex ion and necessary counter ions[Co(NH3)5Cl]Cl2

We can see it has a coordination no. of 6.Complex ion:[Co(NH3)5Cl]2+Co3+ + 5 NH3 + Cl- =1(3+) + 5 (0) + 1(1-) = 2+

Counter ions:2 Cl- CHY4006 GRH*

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Complex IonSpecies where transition metal ion is surrounded by a certain number of ligands.

Transition metal ion:Lewis acidLigands:Lewis bases

Co(NH3)63+Pt(NH3)3Br+CHY4006 GRH*

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LigandsMolecule or ion having a lone electron pair that can be used to form a bond to a metal ion (Lewis base).

coordinate covalent bond: metal-ligand bond

monodentate: one bond to metal ionbidentate: two bond to metal ionpolydentate: more than two bonds to a metal ion possibleCHY4006 GRH*

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Formulas of Coordination Compounds1.Cation then anion2.Total charges must balance to zero3.Complex ion in brackets

K2[Co(NH3)2Cl4]

[Co(NH3)4Cl2]ClCHY4006 GRH*

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NOMENCLATURECHY4006 GRHIn naming salts, the cation is written before the anionWithin a complex ion , the ligands are named before the metal ionLigands are listed in alphabetical orderPrefixes that give the number of ligands are not considered in determining the alphabetical orderThe names of anionic ligands end in the letter oNeutral ligands generally have the molecule name. Exception are water and ammoniaA greek prefix (di,tri,tetra, penta, and hexa) is used to indicate the number of each ligand.

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NOMENCLATURECHY4006 GRHIf the complex is an anion, its name ends in -ateThe oxidation number of the metal is given in parenthesesSome metals which are part of the anion complex will use the latin name with -ate as an endingWhen the name of the ligand has a prefix, use: bis(2), tris (3), tetrakis (4) to give the number of ligands in the compound.*

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Names of Coordination Compounds1.Cation then anion2.Ligands in alphabetical order before metal ionneutral:molecule name*anionic:-ide -oprefix indicates number of each3.Oxidation state of metal ion in () only if more than one possible

4.If complex ion = anion, metal ending -ateCHY4006 GRH*

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CHY4006 GRHCation NameLatin NameAnion NameCopperCuprumCuprateGoldAurumAurateIronFerrumFerrateLeadPlumbumPlumbateSilverArgentumArgentateTinStannumStannate

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CHY4006 GRHAnion NameLigand NameBromide, Br-BromoCarbonate, CO32-CarbonatoChloride, Cl-ChloroCyanide, CN-CyanoFluoride, F-FluoroHydroxide, OH-HydroxoOxalate, C2O42-OxalatoOxide, O2-OxoSulfate, SO42-Sulfato

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CHY4006 GRHMoleculeLigand NameAmmonia, NH3AmmineCarbon monoxide,COCarbonylWaterAquaEthylenediammine, enEthylenediammineEXAMPLES:[Co(NH3)5Cl]Cl2 Pentaamminechlorocobalt(III) ChlorideK4[Fe(CN)6] Potassium Hexacyanoferrate(II)*

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ExamplesK2[Co(NH3)2Cl4]

potassium diamminetetrachlorocobaltate(II)

[Co(NH3)4Cl2]Cl

tetraamminedichlorocobalt(III) chlorideCHY4006 GRH*

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Structural Isomerism 1

Coordination isomerism: Composition of the complex ion varies.

[Cr(NH3)5SO4]Br and [Cr(NH3)5Br]SO4CHY4006 GRH*

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Structural Isomerism 2Ligand isomerism: Same complex ion structure but point of attachment of at least one of the ligands differs.

[Co(NH3)4(NO2)Cl]Cland[Co(NH3)4(ONO)Cl]ClCHY4006 GRH*

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Linkage Isomers[Co(NH3)5(NO2)]Cl2Pentaamminenitrocobalt(III)chloride[Co(NH3)5(ONO)]Cl2Pentaamminenitritocobalt(III)chlorideCHY4006 GRH*

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Stereoisomerism 1Geometric isomerism (cis-trans):

Atoms or groups arranged differently spatially relative to metal ion

Pt(NH3)2Cl2

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Stereoisomerism 2Optical isomerism:

Have opposite effects on plane-polarized light(no superimposable mirror images)CHY4006 GRH*

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VALENCE BOND THEORY OR HYBRIDISATIONBONDING IN TRANSTION METALS*CHY4006 GRH

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Valence Bond TheoryCHY4006 GRHUses hybrid orbitals to hold the donated electron pairs for formation of the coordinatecovalent bondsCan explain the structure and magnetic properties.Select low energy empty metal orbitals to hybridize for the appropriate geometryIf there are not enough orbitals, pair up any unpaired metal electrons to free up orbitals.

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CHY4006 GRHHybrid orbital sets:d2sp3 octahedralsp3 tetrahedraldsp2 square planar*

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[FeF6 ]3-CHY4006 GRH*

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GROUP WORKCHY4006 GRHWrite box diagrams for the electron configuration of Cr(III), using high spin and lowspin arrangements of 6 ligands.Would there be two arrangements for Cr(III)?Consider the electron configurations for CoCo [Ar]3d74s2

What is the valence bond description of Co(NH3)63+?*

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Valence Bond Theory LimitationsCHY4006 GRH

Valence bond theory limitations:Explains, but does not predict.Qualitative explanations; does not explain relative stability. Cannot explain color and spectra. Cannot explain relative stability of structural isomers.*

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Introduction to Crystal Field theoryAssumes that the only interaction between the metal ion and the ligands, is an electrostatic or ionic one.with the ligands being regarded as negative point charges. Though we know this is not entirely the case, the theory does provide some interpretation of the properties of these complexes.

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Crystal Field EffectsIn an isolated metal ion the five d-orbitals are degenerate, ie they are at the same energy level.If they are exposed to a symmetric field of negative charges they will remain degenerate but the energy level would be higher due to repulsion between the negative ligand field and the negative electrons in the d-orbitals.If the field which the d-orbitals are exposed to are from real ligands, such as H2O, as in the case of [Fe(H2O)6]3+, the symmetry of the field will not be spherical and the degeneracy of the d-orbitals will be removed. CHY4006 GRH*

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Octahedral symmetryCHY4006 GRH*

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Splitting of the d orbitalsCHY4006 GRH*

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Colour and the Spectrochemical Seriesimportant aspects of CFT is that not all ligands are identical when it comes to causing a separation of the d-orbitals. There is clear evidence for this from the multitude of colours available for a given metal ion when the ligands or stereochemistry are varied.For octahedral complexes this is a reflection of the energy difference between the higher dz2, dx2-y2 (eg subset) and the dxy, dyz, dxz (t2g subset).CHY4006 GRH*

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In general the difference in energy between the two sets of degenerate orbitals is denoted by .This corresponds to wavelengths of light in the visible spectrum and the colour of complexes can be attributed to electronic transitions between the lower and higher energy sets of d-orbitals. In other words complexes are coloured because the magnitude of corresponds to the energies in the visible region of the electromagnetic specrum.CHY4006 GRH*

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SPECTROCHEMICAL SERIES The SPECTROCHEMICAL SERIES is a list of ligands ranked in order of their ability to cause large orbital separations.When metal ions that have between 4 and 7 electrons in the d orbitals form octahedral compounds, two possible electron distributions can occur. These are referred to as either weak field - strong field or high spin - low spin configurations. Increasing I- < Br- < SCN- ~Cl- < F- < OH- ~ ONO- < C2O42- < H2O < NCS- < EDTA4- < NH3 ~ pyr ~ en < bipy < phen < CN- ~ CO CHY4006 GRH*

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High spinLow spinCHY4006 GRH*

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[V(H2O)6]2+[V(H2O)6]3+[Cr(NH3)6]3+[Cr(NH3)5Cl]2+sCHY4006 GRH*

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Tetrahedral ComplexesCHY4006 GRH*

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Take for example, Fe2+- d6CHY4006 GRH*

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A high spin complex may be defined as one in which the d-orbitals are arranged according to Hunds rule to give the maximum number of unpaired electrons whereas a low spin complex may be defined as one in which the d electrons are paired to give the maximum number of doubly occupied d-orbitals and a minimum number of unpaired electrons.The terms weak field, strong field give an indication of the splitting abilities of the ligand. Water and those ligands to the left such as the halides, always give rise to small splittings of d orbitals for first row transition metal ions and hence are referred to as weak field ligands. Conversely, CN- is a strong field ligand, since it causes large splittings of the d-orbitals.

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Magnetic MomentsFor first row transition metals the magnetic moment may be predicted, for other rows it becomes more complex orbital contributions etc.To predict the magnetic moment, we can use the simple spin-only formula: m = [4S(S+l)] Bohr Magneton (BM)Where S is the spin quantum number (1/2 for each unpaired electron). An alternative representation is: m = [n(n+2)] Bohr Magneton (BM)Where n is the number of unpaired electrons. CHY4006 GRH*

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ExamplesCHY4006 GRH*

K2CuCl4[Fe(H2O)6]3+[Co(CN)6]3-[Co(F)6]3-metal ionCu2+Fe3+Co3+Co3+number of d electrons9566stereochemistrytetrahedraloctahedraloctahedraloctahedralHigh Spin/Low SpinNot relevantHigh SpinLow spinHigh spin# of unpaired electrons1504magnetic moment(3) B.M(35) B.M.0(24) B.M.

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Magnetic Moments of Octahedral, Tetrahedral and Square Planar ComplexesCHY4006 GRH*

Comparison of calculated spin-only magnetic moments with experimental data for some octahedral complexesIonConfigurationmso (B.M.)mobs (B.M.)Ti(III)d1 (t2g1)3 = 1.73 1.6-1.7 V(III)d2 (t2g2)8 = 2.832.7-2.9Cr(III)d3 (t2g3)15 = 3.883.7-3.9Cr(II)d4 high spin (t2g3 eg1)24 = 4.904.7-4.9Cr(II)d4 low spin (t2g4)8 = 2.833.2-3.3Mn(II)/ Fe(III)d5 high spin (t2g3 eg2)35 = 5.925.6-6.1Mn(II)/ Fe(III)d5 low spin (t2g5)3 = 1.731.8-2.1Fe(II)d6 high spin (t2g4 eg2)24 = 4.905.1-5.7Co(II)d7 high spin (t2g5 eg2)15 = 3.884.3-5.2Co(II)d7 low spin (t2g6 eg1)3 = 1.731.8Ni(II)d8 (t2g6 eg2)8 = 2.832.9-3.3Cu(II)d9 (t2g6 eg3)3 = 1.731.7-2.2

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Comparison of calculated spin-only magnetic moments with experimental data for some tetrahedral complexesIonConfigurationmso(B.M.)mobs (B.M.)Cr(V)d1 (e1)3 = 1.73 1.7-1.8 Cr(IV) / Mn(II)d2 (e2)8 = 2.832.6 - 2.8Fe(V)d3 (e2 t21)15 = 3.883.6-3.7Cr(II)d4 (e2 t22)24 = 4.90-Mn(II)d5 (e2 t23)35 = 5.925.9-6.2Fe(II)d6 (e3 t23)24 = 4.905.3-5.5Co(II)d7 (e4 t23)15 = 3.884.2-4.8Ni(II)d8 (e4 t24)8 = 2.833.7-4.0Cu(II)d9 (e4 t25)3 = 1.73-

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The Stability of Metal Complexes and the Chelate EffectA metal ion in solution does not exist in isolation, but in combination with ligands (such as solvent molecules or simple ions) or chelating groups, giving rise to complex ions or coordination compounds.These complexes contain a central atom or ion, often a transition metal, and a cluster of ions or neutral molecules surrounding it. Many complexes are relatively unreactive species remaining unchanged throughout a sequence of chemical or physical operations and can often be isolated as stable solids or liquid compounds. Other complexes have a much more transient existence and may exist only in solution or be highly reactive and easily converted to other species.All metals form complexes, although the extent of formation and nature of these depend very largely on the electronic structure of the metal.

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Thermodynamic StabilityThe "stability of a complex in solution" refers to the degree of association between the two species involved in the state of equilibrium. Qualitatively, the greater the association, the greater the stability of the compound. The magnitude of the (stability or formation) equilibrium constant for the association, quantitatively expresses the stability. Thus, if we have a reaction of the type: M + 4L ML4then the larger the stability constant, the higher the proportion of ML4 that exists in the solution. CHY4006 GRH*

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A complex is called stable if the equilibrium is on the side of the products. Free metal ions rarely exist in solution so M will usually be surrounded by solvent molecules which will compete with the ligand molecules, L, and be successively replaced by them. For example:

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For simplicity, we generally ignore these solvent molecules and write stability constants as follows: l. M + L ML K1 = [ML] / [M] [L] 2. ML + L ML2 K2 = [ML2] / [ML] [L] 3. ML2 + L ML3 K3 = [ML3] / [ML2] [L] 4. ML3 + L ML4 K4 = [ML4] / [ML3] [L] where K1, K2 etc. are referred to as "stepwise stability constants". Alternatively, we can write the "Overall Stability Constant" thus: M + 4L ML4 b4 = [ML4]/ [M] [L]4The stepwise and overall stability constants are therefore related as follows: b4 =K1.K2.K3.K4 or more generally, bn =K1.K2.K3.K4--------------K nCHY4006 GRH*

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If we take as an example, Cu2+ + NH3 Cu(NH3)2+ K1 = [Cu(NH3)2+]/[Cu2+] [NH3]CuNH32+ + NH3 Cu(NH3)22+ K2 = [Cu(NH3)22+]/[Cu(NH3)2+] [NH3]etc. where K1, K2 are the stepwise stability constants. Also: b4 = [Cu(NH3)42+]/[Cu2+] [NH3]4

The addition of the four ammine groups to copper shows a pattern found for most formation constants, in that the successive stability constants decrease. In this case, the four constants are: logK1 = 4.0; logK2 =3.2; logK3 =2.7; logK4 = 2.0 or logb4 =11.9CHY4006 GRH*

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Some other equation to NoteDG = -RTLnb DG = -2.303 RTLog10b DG = DH - TDS

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The Chelate EffectPolydentate ligands are called Chelates because they are able to grasp the metal between two or more donor atoms. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H.D.K. Drew [J. Chem. Soc., 1920, 117, 1456], who stated: "The adjective chelate, derived from the great claw or chele (Greek) of the lobster or other crustaceans, is suggested for the caliper-like groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings. CHY4006 GRH*

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In general chelating ligands form more stable complexes than do related monodentate ligands. The special stability associated with the formation of chelates is called the chelate effect. The chelate effect can be seen by comparing the reaction of a chelating ligand and a metal ion with the corresponding reaction involving comparable monodentate ligands. CHY4006 GRH*

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Monodentate vs BidentateCHY4006 GRH*

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It has been known for many years that a comparison of this type always shows that the complex resulting from coordination with the chelating ligand is much more thermodynamically stable. This can be seen by looking at the values for adding two monodentates compared with adding one bidentate, or adding four monodentates compared to two bidentates, or adding six monodentates compared to three bidentates. CHY4006 GRH*

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Thermodynamic data of the reaction of ammonia and 1,2-diaminoethane (ethylenediamine) with Ni2+CHY4006 GRH*

# of ligandslog bDG (kJmol-1)1 NH32.8-162 NH3 (1 en)5.0 (7.51)-28.5 (-42.8)3 NH36.6-37.74 NH3 (2 en)7.87 (13.86)-44.9 (-79.1)5 NH38.6-49.16 NH3 (3 en)8.61 (18.28)-49.2 (-104.4)

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An alternative view comes from trying to understand how the reactions might proceed. To form a complex with 6 monodentates requires 6 separate favourable collisions between the metal ion and the ligand molecules. To form the tris-bidentate metal complex requires an initial collision for the first ligand to attach by one arm but remember that the other arm is always going to be nearby and only requires a rotation of the other end to enable the ligand to form the chelate ring. CHY4006 GRH*

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If you consider dissociation steps, then when a monodentate group is displaced it is lost into the bulk of the solution. On the other hand, if one end of a bidentate group is displaced the other arm is still attached and it is only a matter of the arm rotating around and it can be reattached again. Both sets of conditions favour the formation of the complex with bidentate groups rather than monodentate groups.CHY4006 GRH*

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Coordination chemistry Reactions of Metal Complexes*CHY4006 GRH

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CHY4006 GRHLigand Substitution Reactions:Octahedral ComplexesInert & Labile Complexeslabile => exchange t1/2 less than 1 min. at RTinert => exchange t1/2 more than 1 min. at RT*

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Substitution reactionsLabile complexes Fast substitution reactions (< few min)Inert complexes Slow substitution reactions (>h)a kinetic conceptNot to be confused withstable and unstable (a thermodynamic concept DGf

Mechanisms of ligand exchange reactionsin octahedral complexesIa if associationis more importantId if dissociationis more important*CHY4006 GRH

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Kineticsof dissociative reactions*CHY4006 GRH

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Kineticsof interchange reactionsFast equilibriumK1 = k1/k-1

k2 > [ML5X]*CHY4006 GRH

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Kinetics of associative reactions*CHY4006 GRH

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Principal mechanisms of ligand exchange in octahedral complexesDissociativeAssociative*CHY4006 GRH

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Dissociative pathway(5-coordinated intermediate)Associative pathway(7-coordinated intermediate)*CHY4006 GRH

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Inert and labile complexesSome common thermodynamic and kinetic profilesExothermic(favored, large K)Large Ea, slow reactionExothermic(favored, large K)Large Ea, slow reactionStable intermediateEndothermic(disfavored, small K)Small Ea, fast reaction*CHY4006 GRH

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Labile or inert?*CHY4006 GRH

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Substitution reactions in square-planar complexesthe trans effect(the ability of T to labilize X)*CHY4006 GRH

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Synthetic applicationsof the trans effect*CHY4006 GRH

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Mechanisms of ligand exchange reactions in square planar complexes*CHY4006 GRH

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Electron transfer (redox) reactions-1e (oxidation)+1e (reduction)Very fast reactions (much faster than ligand exchange)

May involve ligand exchange or not

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Outer sphere mechanism[Fe(CN)6]4- + [IrCl6]2-[Fe(CN)6]3- + [IrCl6]3-[Co(NH3)5Cl]2+ + [Ru(NH3)6]2+[Co(NH3)5Cl]+ + [Ru(NH3)6]3+Reactions ca. 100 times fasterthan ligand exchange(coordination spheres remain the same)

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Inner sphere mechanism[Co(NH3)5Cl)]2+ + [r(H2O)6]2+[Co(NH3)5Cl)]2+:::[r(H2O)6]2+[Co(NH3)5Cl)]2+:::[r(H2O)6]2+[CoIII(NH3)5(m-Cl)rII(H2O)6]4+[CoIII(NH3)5(m-Cl)rII(H2O)6]4+[CoII(NH3)5(m-Cl)rIII(H2O)6]4+[CoII(NH3)5(m-Cl)rIII(H2O)6]4+[CoII(NH3)5(H2O)]2+ + [rIII(H2O)5Cl]2+[CoII(NH3)5(H2O)]2+[o(H2O)6]2+ + 5NH4+*CHY4006 GRH

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Inner sphere mechanismReactions much faster than outer sphere electron transfer(bridging ligand often exchanged)

r = k [Ox-X][Red] k = (k1k3/k2 + k3)Tunnelingthrough bridgemechanism*CHY4006 GRH

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