1Selections from Chapter 14Reactions of Coordination CompoundsCHEM 462December 6thT. HughbanksLigand Substitution Reactions Reminders:kinetic terms: Labile vs. Inertthermodynamic terms: Stable vs. Unstable e.g., a thermodynamically favored, slow reaction:[Co(NH3)6]3+ + 6 H3O+  [Co(H2O)6]3+ + 6 NH4+ ; K = 1025 Ligand substitution, the two extremes: SN1: [L5MX]  [L5M] + X (slow)(dissociative) [L5M] + Y  [L5MY] (fast) SN2: [L5MX] + Y  [L5MXY] (slow)(associative) [L5MXY]  [L5MY] + X (fast)Rate Laws Ligand substitution, the two rate laws: SN2: L5MX + Y  [L5MXY] (slow)(associative) [L5MXY]  [L5MY] + X (fast)rate = k1[L5MX][Y] SN1: L5MX  L5M + X (slow)(dissociative) L5M + Y  [L5MY] (fast)rate = k1[L5MX]2Complications Conjugate base formation (if the complex has acidic protons):e.g. basic catalysis,[Co(NH3)5Cl]2+ + OH–  [Co(NH3)4(NH2)Cl]+ + H2O (fast)[Co(NH3)4(NH2)Cl]+  [Co(NH3)4(NH2)]2+ + Cl– (slow)[Co(NH3)4(NH2)]2+ + Y:–  [Co(NH3)4(NH2)Y]+ (fast)[Co(NH3)4(NH2)Y]+ + H2O  [Co(NH3)5Y]2+ + OH– (fast)(Q1: Does it make sense that the 2nd step is slow?)(Q2: Why does the deprotonated complex more lose Cl– fasterthan the original complex?)rate  [complex][OH–]Solvent Intervention (esp. H2O) Suppose solvent (H2O) intervenes in rate det. step:LnMX + H2O  LnM(H2O) + X (slow)LnM(H2O)  LnMY + H2O (fast)Either step could be associative or dissociative, but eitherway, the obsd. rate  [LnMX](i.e., rate not dependent on [Y] & [H2O] is essentially const.) example: square planar complexes of PtIIobsd. rate =k1[PtL3X] +k2[PtL3X][Y]- both pathwaysare associativePtL3XPtL3XPtL3X+ H2O+ Y:k1k2+ Y:- H2OfastMore ComplicationsIon-Pairs (outer-sphere complex formation):L5MXn+ + Ym-  {[L5MX][Y]}(n-m)+ (ion-pair complex)no bonds made or broken,2nd step: {(L5MX)(Y)]}(n-m)+  L5MY + X (slow)obs. rate  [L5MX][Y]- this is the expected rate law whether or not themechanism is “associative” in the sense that a L5MXYcomplex forms3Mechanistic Clues Activation parameters: H‡, S‡, V‡ How are each of these measured? For dissociative reactions, we expectthat all of these will be > 0.Water Exchange RatesWater Exchange Rates For alkali and alkaline earth, dissociativemechanism seems to hold throughout.rates increase with ionic sizeM-OH2 bonds weaken with increasing ion sizeM-OH2 bonds break in transition stateMIII ions: Sc3+ < Y3+ < La3+Al3+ < Ga3+ < In3+4Trends for T.M.sV2+, Ni2+ > Co2+ > Cr2+, Cu2+ > Fe2+ > Mn2+, Zn2+Does this LFSE trend correlatewith exchange rates?V‡ > 0 for Fe2+, Co2+, Ni2+but, V‡ < 0 for V2+, Mn2+Co3+, Rh3+, Ir3+, Cr3+ all quite inert - all have highLFSE’sAssociative vs. DissociativeV‡ measurements indicate mechanisms may differ inseemingly similar reactions:[Co(NH3)5(H2O)]3+ + Cl–  [Co(NH3)5Cl]2+ + H2OV‡ = 1.4 ± 0.8 cm3 mol-1[Rh(NH3)5(H2O)]3+ + Cl–  [Rh(NH3)5Cl]2+ + H2OV‡ = 3.0 ± 0.7 cm3 mol-1[Cr(NH3)5(H2O)]3+ + Cl–  [Cr(NH3)5Cl]2+ + H2OV‡ = –4.9 ± 0.8 cm3 mol-1Anation Reactions Replacement of water by incoming anion:[ML5(H2O)]2+ + X–  ML5X + H2O General Observations:1. Rate depends on complex, but dependence on incomingligand (if any) involves only concentration (not onidentity of anion).2. rates for anation comparable to water exchangeexplanations for anation of aquo ions?[M(H2O)6]n+ + X–  {[M(H2O)6]X}(n-1)+(outer-sphere complex){[M(H2O)6]X}(n -1)+ + Y:–  {[M(H2O)5]X}(n-1)+ + H2O(slow)rate  [M(H2O)6n+][X–]5Anation of anionic complexes: no ion-pairinge.g.,: [Co(CN)5(H2O)]2–  [Co(CN)5]2– + H2O[Co(CN)5]2– + X–  [Co(CN)5X]3–(pure dissociative mechanism)In some cases, one can distinguish between differentincoming anions:[Co(CN)5(H2O)]2–+ :X–[Co(CN)5]2–+ H2O + :X–[Co(CN)5]2–···X–[Co(CN)5X]3–+ H2O‡depends on X–Substitution Rxns on Sq. Planar Complexes Stereochemistry is preserved!Mechanism: Replacement of C with XDPtIICBA:XDPtIICBAXDPtIICBAXDPtIIXBACDPtIIXBA:C Q: How can traces of free ligand can lead to cis/trans isomerization?Trans EffectLigand exchange reactions can be directed by thelabilizing ability of ligands in trans positionsXPtXL'L:YYPtXL'LXPtYL'Lorproduct depends labilizing abilityTrans effect series:CO, CN–, C2H4 > PR3, H– > CH3– > C6H5–, NO2–,I–, :SCN– > Br–, Cl– > py, NH3 , OH–, H2O6Stereochemical Control, ExamplesClPtClClCl:NH3ClPtNH3ClCl2– –:NH3NH3PtNH3ClClNH3PtNH3H3NH3NCl–NH3PtNH3ClNH32+ +NH3PtClClH3NCl–cistransProblems Give synthetic routes to cis- and trans-[PtCl2(NH3)(NO2)]– Consider a reaction of the type: trans-[PtL2Cl2] + Y  trans-[PtL2ClY]+ + Cl–what complications might arise if the intermediate inthis reaction is has a fairly long lifetime? The reaction below can be carried out in liquidammonia, but is very slow unless a smallamount of KNH2 is added. Why? [Cr(NH3)5Cl]2+ + NH3  [Cr(NH3)6]3+ +