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Reaction Mechanism of d-metal complexes Chapter 20 Inorganic Chem 160:371 November 2010 M. Greenblatt Ligand substitution reactions: one Lewis base (Y) displaces another (X) Y + M-X → M-Y + X e.g., [Co(OH2)6]2+ (aq) + Cl- (aq) → [CoCl(OH2)5]+ (aq) + H2O (l) Both thermodynamic and kinetic effects determine chemical reactionA reaction may be thermodynamically possible (∆G < 0), yet kinetically it is not reactive (nonlabile) Equilibrium, or Formation constants: e.g., coordination equilibrium for ligand substitution is [Fe(OH2)6]3+ (aq) + SCN- → [FeSCN(OH2)5]2+ (aq) + H2O )l) Kf = [FeSCN(OH2)52+] / [Fe(OH2)6 3+][SCN-] Kf is the formation constant of the complex; if Kf is large, incoming L binds Stronger than H2O (solvent); if Kf is small, H2O binds stronger than L Kf spans a huge range, depending on M and L If more than one L is replaced by substitution, then Kf is more complex: [Ni(OH2)6]2+ (aq) + 6NH3 → [Ni(NH3)6]2+ (aq) + 6H2O (l) At least 6 steps are involved: [Ni(OH2)6]2+ (aq) + NH3 (aq) → [NiNH3(OH2)5]2+ (aq) + H2O (l) Kf1 = [NiNH3(OH2)5]2+ (aq) + NH3 (aq) → [Ni(NH3)2(OH2)5]2+ (aq) + H2O (l) Kf2 etc…..to Kf6 in general Kf1 > Kf2>Kf3……>Kf6….. This is understood in terms of ∆G0 = -RTlnKf successive Kf decrease due to decreased # of H2O available for replacement - deminishing statistical factor is reflected in stepwise Kf values for for above reaction Note Kfn/Kfn+1 is not very large In general: M + L → ML Kf1 = [ML]/[M][L] ML + L → ML2 Kf2 = [ML2/[ML][L] MLn-1 + L → MLn Kfn = [MLn/[MLn-1][L] Overall formation constant: M + nL → MLn βn = [MLn]/[M][L]n βn = Kf1x Kf2…..Kfn What are the dissociation constants of MLn? ML → M + L etc A reversal of this trend indicates an electronic, or structural change e.g.: Kfn > Kfn+1 [Fe(bipy)3] 3+ much more stable than [Fe(OH2)2(bipy)2]3+ LFSE of LS, t2g5eg0 >> LFSE of HS t2g3eg2 [Fe(OH2)6]3+ (aq) + bipy (aq) → [Fe(OH2)4(bipy)]3+ (aq) + 2H2O (l) log Kf = 4.2 [Fe(OH2)4(bipy)]3+ (aq) + bipy (aq) → [Fe(OH2)2(bipy)2]3+ (aq) + 2H2O (l) log Kf = 3.7 [Fe(OH2)2(bipy)2]3+ (aq) + bipy (aq) → [Fe(bipy)3]3+ (aq) + 2H2O (l) log Kf = 9.3 Also if Kfn+1 < Kfn some change has occurred Note the reaction of [Hg(OH2)6]2+ + ClKf1 = 6.74; Kf2 = 6.48; Kf3 = 0.85 [HgCl2(OH2)4] (aq) + Cl- (aq) → [HgCl3(OH2)]- (aq) + 3H2O (l) CN = 6 → CN = 4 Chelate effect 1. [Cd(OH2)6 ]2+ (aq) + en (aq) → [Cd(en)(OH2)4 ]2+ (aq) + 2H2O (l) 2. [Cd(OH2)6 ]2+ (aq) + 2NH3 (aq) → [Cd(NH3)2(OH2)4 ]2+ + 2H2O (l) ∆H are similar, ∆S(1) = +13 J(Kmol)-1, ∆S(2) = -5.2 J(Kmol)-1 Chelate complexes are always more stable due to increase of ∆S & kinetic effect ∆G ≡ ∆H-T∆S chelate effect has important applications: porphyrin, edta4- complexing agents, biochemical metal sites very large Kf (1012-1025 indicates chelated complex Examples of chelate comlexes Diamine metal comlex M M Note five membered rings-stable Photon driven oxidation system to generate O2 from H2O with Solar energy λ-MnO2 Catalyst eRuII*(bpy)3 O2 + 4H+ hυ S2O82- 2H2O SO4- + SO42- RuII(bpy)3/RuIII(bpy)3 Eox = 1.4 V e- Illumination was done using 250W industrial light source with UV filtered by Pyrex and IR with a 12 cm path water filter at intensity of 20 mWcm-2. Irving-William Series: summarizes relative stabilities of M2+ complexes and reflects electrostatic and LSE effects Ba2+<Sr2+<Ca2+<Mg2+<Mn2+<Fe2+<Co2+<Ni2+<Cu2+>Zn2+ Note sharp increase in Kf for Fe2+, d6, to Cu2+ d9 LSE decrease for Zn2+, d10 (LSE=0) Why Kf (Cu2+ complex) > Kf (Ni2+ complex)? Ligand substitution reactions MLxX + Y → MLxY + X X is the leaving group and Y is the entering group Kinetically complexes are inert and labile Metal complexes that react with t1/2 ≤ 1 min are kinetically labile If the reaction is significantly longer than this, it is considered kinetically nonlabile, or inert No relationship between thermodynamic stability and lability towards substitution. e.g., ∆hydGº of Cr3+ and Fe3+ are similar, [Cr(H2O)6]3+ d3 undergoes substitution slowly, while Fe(H2O)6]3+ d5 fast Overall formation constant of [Hg(CN)4]2- is greater than that of [Fe(CN)6]4Hg(II) complex is kinetically labile and exchanges CN rapidly (with isotopically labelled CN, while Fe(II) d6 HS slowly; d3 and d6 complexes are extra stable due to LFSE Average residence time, τ = 1/k of H2O in first coordination sphere of a metal ion Note huge range for d-metals: function of dn e.g., [Cr(H2O)6]3+ d3, [Rh(H2O)6]3+ LS d6 LFSE k [M(OH2)x]n+ + H2O ←→ [M(OH2)x-1 (OH2)]n+ + H2O rapid Rate of H2O exchange = xk[M(OH2)xn+] [Ir(H2O)6]3+ slow, τ = 109 τ = 290 years Lanthanides are large ions, k > 107 Main group k for H2O exchange Increases with: increasing size of M increasing coordination # decreasing surface charge density (Z/reff) Some generalizations about reactivity: 1.Metal comlexes without extra stability (e.g., LFSE & chelating L, are labile) 2.Very small ions are less labile, because of strong M-L bond and steric effects (difficult for Y to approach M) 3. All complexes of s (except Be2+ and Mg) are labile) 4. Complexes of M(III) f-block are all very labile 5. Complexes of d10 ions (Zn2+, Cd2+, Hg2+) are normally labile 6. Across d series M2+ are generally labile with distorted Cu2+ most labile 7. Across d series M3+ are distinctly less labile than M2+ 8. d-M comlexes with electronic configurations with d3 and d6 (Cr(III), Co(III) are nonlabile due to large LFSE, chelate complexes, like [Fe(dipy)]2+ are even more nonlabile 9. 4d, 5d metal complexes are usually nonlabile, because of large LFSE (LS) 1. Identities of Ls effect rates of reaction; incoming L has greatest effect on rate 2. Keq of displacement reaction can rank Ls in order of their strength as Lewis bases 3. For kinetics, concept of nuclephilicity is used (instead of equilibrium concept of basicit 4. Nuclephilicity: rate of attack on a complex by a Lewis base elative to the rate o attack by a reference Lewis base Mechanism of reactions – sequence of elementary steps by which the reaction takes place often not all the steps can be determined, only the slowest step, the rate determining step First step in elucidating mechanism is determination of the rate law - how rate changes with concentration of reactants e.g., [Ni(OH2)6]2+ (aq) + NH3 (aq) → [Ni(OH2)5]2+ (aq) + H2O Rate = k[Ni(OH2)62+][NH3] Generally the slowest elementary step of reaction controls the overall rate of raction and the overall rate law – this is the rate-determining step Mechanism is usually proposed: it must be consistent with all the experimental facts. A mechanism often cannot be proven, since another mechanism may also be consistent with the experimental data. For substitution reactions, square planar and octahedral complexes will be considered only Stoichiometric equations often say nothing about mechanism [(H3N)5Co(CO3)]+ + 2H3O+ → [(H3N)5Co(H2O)]3+ + CO2 + 2H2O This might suggest direct substitution of CO32- by H2O However, use of H218O solvent shows that all the O in the [(H3N)5Co(H2O)]3+ complex comes from CO32- 2+ Proposed mechanism: H + H2O [(H3N)5Co(OCO2)] + H3O+ → [(H3N)5Co— O CO2 ↓ [(H3N)5Co(OH)]2+ + CO2 + H2O ↓ H3O+ [(H3N)5Co(H2O)]3+ + H2O Classification of mechanisms for nucleophilic substitutions dissociative, associative and interchange Dissociative (D): MLxX → {MLx } + X intermediate leaving group {MLx } + Y → MLxY entering group Typical profile of a reaction with D mechanism e.g., W(CO)6 → W(CO)5 + CO W(CO)5 + PPh3 → W(CO)5Ph3 The intermediate W(CO)6 has been isolated Associative: MLxX + Y → {MLxXY} entering group {MLxXY} → MLxY + X intermediate leaving group This mechanism in square planar Complexes of d8 M (Ni(II), Pd(II), Pt(II), Ir(I) e.g., [Ni(CN)4]2- + 14CN- → [Ni(CN)4 14CN]3[Ni(CN)4(14CN)]3- → [Ni(CN)3(14CN)]2- + CNIntermediate was isolated Interchange (I) mechanism MLxX +Y → Y……MLx……X → MLxY + X Transition state In most substitution pathways: bond formation with Y and bond breaking with X is concurrent. In the I (interchange) mechanism: no intermediate phase but various transition states: Difference between A and I is life time of intermediate state if it is long enough, and can be detected, A e.g., [Ni(CN)5]3- trigonal bipyramid was observed experimentally and isolated in the solid state – intermediate in square planar substitution reaction trans and cis transformation also evidence for trigonal bipyramidal intermediate MLxX +Y → Y……MLx……X → MLxY + X Transition state n most substitution pathways: bond formation with Y and bond breaking with X is concurrent. In the I (interchange) mechanism: no intermediate phase but various transition states: dissociative interchange (Id) bond breaking dominates bond forming associative interchange (Ia) bond formation dominates over bond breaking In associative (A) and Ia the reaction rate shows a dependence on the entering group (Y); in dissociative (D) and Id very small dependence on entering group. In general it is difficult to distinguish between A and Ia and D and Id. An interchange mechanism is a concerted process in which there is no intermediate species with a coordination number different from that of the starting complex Rate determining step is Associative, if rate strongly depends on the incoming L Typical of square planar substitution reaction of d8 metal complexes [PtCl(dien)]- (aq) + I- (aq) → [PtI(dien)]+ (aq) + Cl- (aq) Rate with Br- increases rate by an order of magnitude Profile of mechanisms with associatively activated steps: a. Associative mechanism associatively activated, Aa - formation of MLnXY is rate determining step b. Dissociative mechanism, associatively activated - Da formation of MLnY is rate determining step Profile of mechanisms with dissociatively activated steps: rate is largely independent on identity of Y (substitution in octahedral comlexes) Dissociative mechanism associatively activated, Ad if loss of X in MLnXY is rate determining step b. Dissociative mechanism, dissociatively activated - Dd if initial loss of X from MLnX is rate determining step Profile of Interchange mechanisms Can be either associative or dissociative a. Interchange mechanism associatively activated, Ia if rate depends on formation of M….Y bond b. Interchange mechanism, dissociatively activated - Id if rate depends on rate at which M…X bond breaks Eyring Eq.- f (T, ∆H#, ∆S#) k = (k’T/h)e-∆G#/RT = k’T/h)e [-∆H#/RT +∆S#/R] Linearized form: ln(k/T) = -∆H#/RT + ln(k’/h) + ∆S#/R Pg 765 transition state k = rate constant ∆H# =enthalpy of activation (J/mol) ∆S# = entropy of activation (J/mol-K) k’ = Boltzman const h= Planck’s const. From Eq: ln(k/T) = -∆H#/RT + ln(k’/h) + ∆S#/R Plot of ln(k/T) vs 1/T (Eyring plot) Large negative ∆S# indicative associative Pressure dependence of k→ ∆V# Volume of activation (cm3/mol) d(lnk)/dP = -∆V#/RT k = rate const. P = pressure ∆V# =volume of activation (Vtran-Vinit) R = molar gas const. T = temperature (K) integrated form: Ln[(k(P1)/k(P2)] = (-∆V#/RT)(P1- P2) negative value of ∆V# ⇒ associative positive value of ∆V# ⇒ dissociative Substitution in Square Planar complexes d8 configuration, Rh(I), Ir(I), Pt(II), Pd(II), Au(III) form square planar comlexes Many kinetic studies on nucleophilic substitution of Pt(II) square planar complexes indicate that the mechanism is associative or Ia : ∆S# and ∆V# are negative k for the substitution of Cl- by H2O in [PtCl4]2-, [PtCl3(NH3)]-, [PtCl2(NH3)2], and [PtCl(NH3)3]+ are similar, suggests an associative mechanism Substitution in a Pt(II) square palanar complex: PtL3X + Y → PtL3Y + X Experimental rate law: Rate = -d[PtL3X]/dt = k1[PtL3X] + k2[PtL3X][Y] suggests that reaction proceeds simultaneously by two routes: If reaction is studied under psuedo-first order conditions, S=olvent Y and S are in large excess: [Y]t ~ [Y]0; [S]t ~ [S]0 Rate= -d(PtL3X)/dt = kobs[PtL3X]; kobs = k1 +k2[Y] Rate = -d(PtL3X)/dt = k1[PtL3X] + k2[PtL3X][Y] Rate= -d(PtL3X)/dt = kobs[PtL3X] kobs = k1 +k2[Y] k2 from associative mechanism is effected by Y Study k vs [Y] (Y in large excess; conc. ~const.) Here solvent for both is CH3OH With different solvent, k1 intercept, differen Effect of solvent on k1: in polar solvents k1 dominates; in apolar S contribution of k1 is diminished, hence S participates in reaction, so Rate = -d(PtL3X)/dt = k3[PtL3X][S] + k2[PtL3X][Y] k1 =k3[S] Nucleophilicity of Y f for a specific complex is given by the nucleophilicity parameter nPt = log k2[Y]/k20 k2[Y] is the second-order rate const for trans-[PtCl2(py)2] + Y → [PtYCl(py)2]- + ClAnd k20 is the rate const when Y is CH3OH Note: if nPt is large, Y is highly nucleophilic nPt seems to correlate with softness of base Cl- < I-; NH3 < P Rate = -d(PtL3X)/dt = k1[PtL3X] + k2[PtL3X][Y] (25.9) k2 arises from associative mechanism, attack of Y on PtL3X, and is dominant when Y is a good nucleophile; but k1 term, which may indicate a concurrent dissociative pathway; in polar solvents k1 becomes dominant, its contribution diminishing in non-polar solvents Thus solvent participates, and rate eq. (25.9) is better written as: Rate = -d(PtL3X)/dt = k3[PtL3X][S] + k2[PtL3X][Y] Since S is in excess, [S] = constant, and k1 = k3[S] When S is a potential L like H2O, it competes with Y entering group in the rate determining step, and X is displaced by either Y or S: k2 PtL3X + Y → PtL3Y + X competes with: k1 PtL3X + S → PtL3S + X fast PtL3S + Y → PtL3Y + X this is non-rate determining Further evidence that both k1 and k2 terms are associative in square planar substitution reactions, is that as bulkiness of Y or S (L) increases, both rate constants decrease Substitution at square planar complexes is stereo-retentiveentering group takes the coordination site of the leaving group Trigonal bipyramid intermediate Further evidence for trigonal bipyramid intermediate The trans effect: the choice of the leaving group in a square planar complex is determined by the nature of the ligand trans to it, and is kinetic in origin NH3 Cl {Cl NH3 3→ Pt —Cl }2-→ {Cl —Pt — Cl}- NH → NH 3→ Cl {Cl —Pt — NH3} Cl Cl NH3 − {NH3—Pt — NH3}2+ Cl→ NH3 cis Cl Cl − {NH3 —Pt — NH3]+ Cl→ {NH3 —Pt — NH3} NH3 Cl trans Preparation of cis- and trans-[PtCl2(NH3)2] by different routes, illustrates the Trans effect One contributing factor to the trans effect is the trans - influence (Box 23.9) L and L’ compete for electron density-use same metal orbitals (dz2 and pz) for M-L and ML’ bonds L | X—M—X | L’ Cl | Cl—Pt—Cl | Cl Pt-Cl (pm) 231.6 }2- The existence of ground-state trans influence (the effec of L on M-L’ bond) is established by solid state structure, vibration (IR) and NMR data for a series of related complexes H- exerts H2C = CH2 } _ ↓ Cl—Pt—Cl b | a Cl PMe3 | Cl —Pt—PMe3 | Cl a=232.7; b=230.5 2041 -7.8 2156 -12.7 2178 -15.6 | Cl— Pt —H | Cl a = 237 IR and 1H NMR data for trans-[PtXH(PEt3)2] XCNIBrν(Pt-H)cm-1 δ(1H for Pt-H) Strong trans-influence PEtPh2 a= 242 Pt-H bond is weakest for X=CN- trans-influence of X-: Cl- E = hν (IR; 1/λ) CN- > I- > Br- >Cl2183 δ→ higher H, lower ν -16.8gβH = hν The trans - influence is not the same as the trans –effect The former is a ground state phenomenon - structural trans-effect the latter is a kinetic effect – kinetic trans-effect The second factor, responsible for the kinetic origin of the trans - effect is π bonding in the 5-coordinate transition state, or intermediate as shown: L2 and M can only communicate electronically via π-bonding if they all lie in the same plane in the transition state. This implies that the transition state must be trigonal bipyramidal, rather than square pyramidal If L2 is a strong π-acceptor, such as CO, CN- or H2C=CH2 it will stabilize the transition state by accepting electron density that the incoming nucleophilic ligand (Y) donates to M and thereby facilitate substitution at the site trans to it General order of the trans - effect: the general ability of ligands to direct trans – substitution spans a factor of ~106 in rates and is: H2O ≈ OH- ≈ NH3 ≈ py < Cl- < Br- < I- ≈ [NO2 ]- < Ph- < Me- < PR3 ≈ H- << CO ≈ [CN]- ≈ C2H4 Trans-effect is useful in synthesis of Pt(II) cis or trans complexes (see 26.19, 26.20) Ligand Nucleophilicity Substitution in Pt(II) complexes depends on nucleophilicity of Y the rate constant k2 (Eq. 26.12) Rate = -d[PtL3X]/dt = k1[PtL3X] + k2[PtL3X][Y] (26.12) increases in the order of the nucleopholicity sequence: H2O < NH3 ~ Cl- < py < Br- < I- < CN- < PR3 Nucleophilicity parameter nPt = log k2/k’2 or nPt = logk2 – logk’2 k’2 is the rate constant for: trans-[PtCl2(py)2] + CH3OH ⇒ trans-[PtCl(py)2CH3OH]+ + ClnPt = = for Y = CH3OH The nucleophilicty parameter, nPt describes the dependence of the rate of substitution in a square planar Pt(II) complex on the nucleophilicy of the entering group, Y trans-[PtCl2(py)2] + CH3OH ⇒ trans-[PtCl(py)2CH3OH]+ + Cl- 26.21 A linear relation of log k2 vs nPt for Pt(II) square planar comlexes of Y is observed for the generally, reaction: PtL3X + Y ⇒ PTL3Y + X Equation of the straight line is defined by: log k2 = s(nPt) + logk’2 s = nucleophilicity discrimination factor logk’2 = rate when Y = CH3OH Ligand substitution in Octahedral complexes H2O exchange in [M(H2O)6]n+ complexes [M(H2O)6n+ + H2(17O) → [M(H2O)5{H2(17O)}]n+ + H2O with M an s, p or d metal, this reaction can be studied with 17O NMR First order rate constants for the above exchange show the following trends 1. For s and p metals k increases with increasing ionic radius, r+ 2. For similar r+ (Li+, Mg2+, Ga3+) increase of charge, slows down reaction 3. For M2+ d-metals, no correlation of k with r+, but there is with dn 4. Limited data for M3+ d ions support behavior as in 3 associative dissociative associative Suggests that bond breaking becomes less (and bond making more) important for d3 –to-d5 configuration For first row d-Mn+ ions (all HS) k for H2O exchange varies greatly: Cr2+ d4 and Cu2+ d9 are kinetically very labile (k ≥ 108 s-1) Cr3+ d3 is kinetically inert (k ≈ 10-6 s-1) V 2+ d2 k ≈ 102 s-1, less labile than later M2+ ions. The rates of water exchange in HS hexa-aquao complexes follows the sequence: V2+ < Ni2+ < Co2+ < Fe2+ < Mn2+ < Zn2+ < Cr2+ < Cu2+ and Cr3+< V3+ < Fe3+ < Ti3+ For a series of ions of similar charge, size, undergoing the same reaction of the same mechanism, it can be reasonably assumed that the collision frequencies and ∆S# are approx. constant and the variation in rate is attributed to ∆H# , which is dependent on loss or gain of LFSE. Model for change of LFSE in going to 5- (dissociative) or 7- (associative) coordinated complex loss of LFSE means increase in ∆H# and slower k V2+ < Ni2+ < Co2+ < Fe2+ < Mn2+ < Zn2+ < Cr2+ < Cu2+ Despite the qualitative nature of CFT/LFSE, good agreement between labality and LFSEs for example for either model, Cr2+ and Cu2+ are most labile due JT The Eigen - Wilkinson Mechanism of ligand substitution in an octahedral complex Water exchange is always more rapid than substitution with other ligands: ML6 +Y → products For most ligand substitutions in octahedral complexes, experimental evidence supports dissociative mechanism; two limiting cases are observed: •at high conc of Y, rate is independent of [Y], pointing to disociative mechanism •at low conc of Y, the rate depends on [Y] and [ML6], indicative of associative mechanism These apparent contradictions are explained by The Eigen - Wilkinson Mechanism: An encounter complex is formed between the substrate and entering ligand in a pre-equilibrium step, followed by loss of a leaving ligand in a rate-determining step Eigen-Wikinson mechanism [Ni(OH2)6]2+ + NH3 → [Ni(OH2)5 (NH3)]2+ + H2O 1st step diffusion of reactants to form a pre-equilibrium pair [Ni(OH2)6]2+ + NH3 ↔ {[Ni(OH2)6]2+, NH3} τ ~1 ns and in general: KE ML6 +Y ⇔ {ML6,Y} Encounter complex KE = {ML6,Y}/ [ML6][Y] KE can be rarely determined experimentally, sometimes can be calculated/estimated theoretically 2nd step is the rate determining step in the Eigen - Wilkinson Mechanism {[Ni(OH2)6]2+, NH3} → [Ni(OH2)5(NH3)]2+ + H2O in general k {ML6 ,Y} → ML5Y + L Rate = k[{ML6,Y}] Cannot just use from KE = {ML6,Y}/ [ML6][Y] [{ML6,Y}] = KE [ML6][Y] as [ML6] is less by the amount that is in the encounter pair Rate = k [{ML6,Y}] [{ML6,Y}] cannot be measured directly, but KE can be estimated KE = [{ML6,Y}] / [ML6] [Y] [M]total is the total concentration of the complex [M]total = [ML6] + [{ML6,Y}] [M]total = [ML6] + KE [ML6] [Y] = [ML6] (1+KE[Y]) [ML6] = [M]total / (1+KE[Y]) thus substituting in the rate expression above [{ML6,Y}] =KE[ML6][Y] Rate = k KE [M]total [Y] / (1+KE [Y]) Rate = k KE [M]total[Y] / (1+KE[Y]) at low conc. [Y] << 1, K[Y] << 1 Rate = k KE [M]total[Y] = kobs [M]total[Y] kobs = kKE kobs is measured experimentally, KE can be measued, or is estimated by theory so k = kobs/KE Rate = k KE [M]total [Y] / (1+KE [Y]) At high conc. of Y, e. g., when Y is the solvent (e.g., H2O exchange) KE[Y]) >> 1 Rate = k [M]total (first order rate eq.) kobs = k Thus reaction with solvent can be compared with other Y, without needing to know KE Further exp. evidence that ligand substitution in octahedral complexes is D or Id is supported by many examples: The rate of ligand substitution depends on the nature of the ligand leaving [Co(NH3)5X]2+ + H2O → [Co(NH3)5H2O]3+ + XFor above reaction, rate of substitution increases as: OH- < NH3 ≈ NCS- < MeCO2- < Cl- < Br- < I- < NO3- Rate constant, k for the substitution reaction: R a t e c o n s t n t s [Ni(H2O)6]2+ + Y → [Ni(H2O)5Y]2+ + H2O The fact that k varies so little is consistent with a Id mechanism if the path was associative, k would depend on nature of Y more f The substitution of an uncharged ligand (H2O) by an anionic one (F-) o is called anation r t OH- < NH3 ≈ NCS- < MeCO2- < Cl- < Br- < I- < NO3- This trend correlates with M-X bond strength; the stronger the bond, lower the rate and consistent with dissociative mechanism with bond breaking the rate-determining step Leaving group effect -the identity of X has large effect on dissociatively activated reaction – rate depends on M….X : lnk = lnK + c; plot of lnk vs lnK 2+ 3+ [Co(NH3)5X] + H2O → [Co(NH3)5H2O] +X k is rate const for forward react., K is equilibrium const ∆G# ∝ -log k LFER with a slope < 1 has associative character ∆G0 ∝ -log K as indicated for analogous Rh(III) complexes ∆G# = p∆G0 + c For Co(III) rate goes as I- > Br- > Cl- for P ≈ 1, i.e., For Rh(III) rate is reversed I- < Br- < Clslope ≈ 1 Co(III) is hard, forms weaker bond with I- than Clslope = 1.0 Rh(III) is softer, forms weaker bond with Cl- than I- this linear relationship between log k and log K represents linear relationship between ∆G# and ∆G0 called linear free energy relationship (LFER) Effect of changing X to X’ on ∆G# & p∆G0 ∆G# = p∆G0 + c linear f(X) with slope, p ~ 1 and shows that for D or Id X has the same effect on the formation of M…X transition state as it has on ∆G0 for the complete elimination of X- Spectator ligands effect the rate of substitution in octahedral complexes by the strength M – L interactions trongest donor ligands increasing the rate, by stabilizing the transition state In Co(III) and Cr(III) both cis and trans L effect rate of substitution in proportion to strength of M-L bond, but no important trans effect e.g., [NiXL5]+ + H2O → [NiL5OH2] + H2O Much faster with L = NH3 than L = H2O, because NH3 is better σ donor, large electron density on M enhences M….X break more readily also larger electron density on M stabilizes reduced coordination of transition state Steric crowding favors dissociative activation formation of transition state relieves crowding Effect of Spectator Ligand Stereochemistry of substitution-square-pyramidal intermediate leads [CoX(en)2 + H2O ⇒ [Co(H2O)(en)2 both cis and trans were studied A]+ A]2+ + X- retention of original geometry trygonal-bipyramidal intermediate can lead to isomerization Table shows that cis comlexes remain cis trans comlexes undergo isomerization to cis in order: A = NO2- < Cl- < NCS- < OHThis can be understood in terms of Id for the two different 5-coordinate intermediate possible Reaction through a square-pyramidal complex leads to retention of geometry Good π -donor L(A) trans to X, favor isomerization Reaction through trigonal-bipyramidal complex can lead to isomerization cis octahedral complex leads to square-pyrmidal transition state trans octahedral complex leads to trigonal-bipyramidal transition state Good π -donor L in equatorial position favor trigonal-bipyramidal Electron -Transfer Processes simplest involve only transfer of electrons and can be monitored by isotopic tracers [56Fe3+(CN)6]3- + [59Fe2+(CN)6]4- → [56Fe2+(CN)6]4- + [59Fe3+(CN)6]3two classes of electron transfer reactions were defined by Taube (Nobel Prize in chemistry 1983) in an outer sphere mechanism, electron transfer occurs without a covalent linkage formed between the reactants in an inner-sphere mechanism, electron transfer occurs via a covalently bonded bridging ligand Kinetic data can sometimes distinguish between these two cases, but often, it is not possible to distinguish between inner- and outer-sphere mechanism