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Reaction Mechanisms of Inorganic Compounds C21J, 4 Lectures Mohammed Bakir, Office #8 Introduction Ligand substitution reactions Octahedral Complexes Square Planar Racemization Reactions Reference: Chapters 9 Textbook: B. Douglas, D. H. McDaniel, & J. J. Alexander, Concepts & Models of Inorganic Chemistry, 2nd edition, Wiley, New York Some Chemistry of Transition Metals Ligand Exchange Photochemical Coordination to metals Redox Reactions Ligand Reactivity Rearrangement of coordination Sphere What are reaction mechanisms? Detail step-by-step analysis of reaction pathway. i.e. sequence of reactions involved in the conversion of reactants to products Rate Law ‘Intimate’ Mechanism refers to details of the mechanism at the molecular level. Consider the reaction: MWL W Reaction Profile M + L W ML W [ML]* Collisions of >Energy [ML] Activated Complex Transition State M+L ML Reaction Coordinate Potential Energy Potential Energy Activation Energy α ∆G‡ [ML]** Intermediate ML [ML]* W Activated Complex Width related to entropy Narrow for –∆ ∆S Wide for + ∆S [ML]* [ML]** M + L [ML] ML Reaction Coordinate Microscopic Reversibility Microscopic reversibility: at equilibrium both forward and reverse reactions proceed at equal rates backward and forward and along the reaction coordinate. Transition State Theory Rate = k[L][M] k α K‡ α exp (-∆G‡/RT) k α exp (∆S‡/R) exp (-∆Εact/RT) Fast reactions are favored by: ∆S‡ more positive. ∆Eact less positive. Note since k is exponentially dependent on ∆S ‡ and ∆Eact. Small changes in k are not significant in the interpretation of the rate law. Type of Reactions Ligand Substitution reactions: are those in which the coordination sphere around the metal changes due exchange of one ligand (lewis base) with another ligand. e.g. Octahedral ML6 + Y W ML5Y + L [Cu(H2O)6]2+ + conc. HCl W[CuCl4]2seconds [Cr(H2O)6]3+ + conc. HCl W NO Reaction Square Planar ML4 + YW ML3Y + L fast within Examples of Substitution Reactions W Substitution reaction [Cu(H2O)4]2+ +NH3 -NH3 W Cu2+ + H2O 4H2O + [Cu(NH3)4]2+ [Cu(NH3)4]2++HCl+H2O → [Cu(NH3)3(H2O)]2+ + NH4+ H 2O Fast reactions [Cu(H2O)4]2+ + Cl- [Cu(H2O)3(Cl)]+ Example [Co(NH3)6]3+ + H2O → [Co(NH3)6(H2O)]3+ Slow several days Lability and inertness: kinetic terms & unrelated to thermodynamic stability of compound and they refer to how quickly a reaction system reaches equilibrium. Inertness is kinetic stability Examples M(n-y)- + nCN- W [M(CN) ] n y Kf = [M(CN)n]y/[Mn-y)[CN]n Compound Kf Exchange Rate [Ni(CN)4]2[Hg(CN)4]2[Fe(CN)6]3[Fe(CN)6]4[Pt(CN)4]2- 1030 1042 1044 1037 ~1040 very fast very fast very fast very fast t1/2 = 1 min. → [Hg(CN)4]2- Thermodynamically stable but kinetically labile, the same for [Ni(CN)4]2i.e. in solutions ligand exchange is fast but always with CN-. Thermodynamics versus Kinetics Thermodynamics → Stability (stable/unstable)→ Kf Formation Constant Kinetics →lability and inertness →rate of replacement of ligands (bases) Ligand Substitution Reactions in Octahedral Compounds Kinetics of H2O exchange LnM(H2O) + H2O*W LnM(H2O*) + H2O (see Figure 1) Kinetics of Water Exchange Li+Na+K+ Rb+Cs+ Be2+ Al3+ Ln3+ Ru2+ V2+ 100 102 Mg2+ Ca2+Sr2+Ba2+ Ln3+Zn2+ Cd2+ Hg2+ Ni2+ 104 Co2+ Fe2+ Mn2+ 106 108 Exchange rate for metal aqua complexes Figure 1 Cu2+, Cr2+ 1010 Langford & Gray Classes for Water Exchange Class I Very fast (diffusion control), k >108 sec-1. The ions in this class are those of alkali metals and alkaline earths (except for Be2+ & Mg2+), Group IIB (except for Zn2+), Cr2+ and Cu2+. Class II Exchange-rate constants are between 104-108 sec-1. The divalent first row transition metal ions (except for V2+, Cr2+ & Cu2+) as well as Mg2+ and the trivalent lanthanide ions. Class III Exchange-rate constants are between 1 and 104 sec1. This class includes Be2+, V2+, Al3+, Ga3+ and several trivalent first-row transition metal ions. Class IV Exchange rate constants between 10-6 to 10-1 sec-1. Member of the set are Cr3+, Co3+, Rh3+, Ir3+ and Pt2+. Inert. Factors that affect water exchange substitution reactions Ionic potential = q/r; charge & size of metal ions Ions with high ionic potential → slow reactions suggesting that the main contribution to the activation energy is the breaking of bond to the leaving group. d-electronic configuration →metal ions in strong LFSE → slow substitution Oh dx2-y2 6Dq dz2 eg 4Dq dxy dxz dyz t2g Spectroelectrochemical Series CN->phen~NO2>en>NH3~py>H2O>C2O42- >OH->F->S2>Cl->Br->IIn strong field metal ions in d3, d4, d5, & d6 are inert i.e. HIGH LFSE Oh → high spin d8 such as NiII & low spin d8 such as Rh(I), Ir(I), Ni(II), Pd(II), Pt(II) & Square planar Au(III) are labil. React faster than d6 & d3 For Cu2+ & Cr2+ the fast exchange is due to Jahn-Teller distortion in the ground state which makes the axial bonds longer than the equatorial bonds. Mechanism of Ligand Substitution in Oh Compounds k1 ML5X W k-1 k2 [ML5] + X Potential Energy ML5X + Y → ML5Y + X Where X is leaving group Y is entering group Consider the following mechanism Wide ∆S + ML5X [ML5] + Y → ML5Y ML Y [ML ] Dissociative pathway-D-type Reaction Coordinate Key: M-X bond breakage, [ML5] is intermediate 5 5 Factors that affect the Strength of M-X Bond δ + δ M -X Electrostatic Charge on metal Center-increase positive charge, M-X bond strengthen Size of metal center-increase size of metal center, M-X bond weaken Charge of leaving group, increase negative charge on X, M-X bond strengthen. Size of leaving ligand, increase size of leaving group, MX bond weaken Charge on other ligands (spectator), increase negative on other ligands, M-X bond weaken Size of spectator ligands, increase size of other ligands, M-X bond weaken Rate Law for Dissociative Mechanism Rate = d[L5MY]/dt =k2[L5M][Y] Apply the steady-state approximation to L5M d[L5M]/dt = 0 =k1[L5MX] – k-1[L5M][X] – k2[L5M][Y] ∴ [L5M] = k1[L5MX]/(k-1[X]+k2[Y]) & Rate = (k1k2[L5MX][Y])/(k-1[X] + k2[Y]) Bimolecular Dissociative Mechanism, D-type If k2[y]>>k-1[X]; i.e. pusedo 1st order ∴ Rate = k1[LnMX] unimolecular e.g. aquation and anation of inert d6 CoIII compounds of the type [Co(NH3)5X]2+. For this mechanism [L5M] is an intermediate that has an appreciable lifetime and can be detected. Note true examples of D-type are not common. Aquation or acid hydrolysis Aquation or acid hydrolysis L5MXn+ + H2O→ [L5M(H2O)]n+1 +XAnation [L5M(H2O)]n+1 + Y- →L5MYn+ + H2O ∆ -X L5M…...X … L5M-Y Y → Activation Energy Vibrational Energy +Y …..…. L5M-X → L5M—X → L5M….X Y The rate is insensitive to the identity of Y, Implication is that both entering and leaving groups are weakly bound in the transition state. This is known as Dissociative interchange (Id). No stable intermediate is observed Plausible Mechanism for Id Dissociative Interchange Narrow, ∆S – L5MX + Y (L5MX,Y) W k2 → fast → (L5MX,Y) (L5MY,X) ML5X +Y [ML5Y,X] ML5Y (L5MY,X) The magnitude of K depends on the ionic charge being larger when Y and the metal complex are of opposite charge. The likelihood of Y present in the outersphere depends on [Y], as indicated in step 1. L5MY +X For short lived Intermediate, or TS Potential Energy K Reaction Coordinate Rate Law for Dissociative Interchange The experimental parameter accessible here is the initial concentration of the reactant complex [L5MX]0 which in solution containing Y exists partly in the form of outersphere complex [L5MX]0 = [L5MX] + [L5MX,Y] K = k1/k2 = [L5MX,Y]/[Y][L5MX] ∴ [L5MX,Y] = K[Y][L5MX] & [L5MX]0 = [L5MX] + K[Y][L5MX] Hence [L5MX] = [L5MX]0/(1 + K[Y]) Rate = -dL5MX/dt = k2[L5MX,Y] Rate = k2K[L5MX]0[Y]/(1 + K[Y]) Associative (a) Mechanism L5MX + Y k2 k1 W k-1 L5MXY L5MXY → L5MY + X Rate = k1[L5MX][Y]/(k-1 +k2) No example for Oh Associative Interchange (Ia) Mechanism . L5M-X → L M — X → L M … .X 5 5 ∆ Activation Energy Vibrational Energy … +Y Y → -X L5M-Y L5M…...X Y Key: M-Y bond starts to form before M-X bond starts to cleave. The entering group (Y) is strongly bound, and rate is sensitive to Y. Implication is that the entering group and leaving group are strongly bound in the transition state. This is known as Associative interchange. (Ia) Summary Inorganic Reaction Mechanisms d a Ia SN2(lim)* Stoichiometric SN1 & SN2 are not good models for inorganic reaction mechanisms.* d SN1(lim) Id SN1 a SN2 intimate Summary L5MX + Y W L5MY + X Rate = a[L5MX][Y]/(1 + b[Y]) Limiting conditions: 1. b[Y] >>1; ∴ the reaction should be first order. 2. b[Y]<<1; ∴ the reaction should be second order. 3. b[Y]=1 ∴ the reaction should be of mixed order. A always second order as long as the final step is not reversible. Substitution inert ions Co3+, Cr3+, Rh3+, Ir3+, Pt4+ & Ni2+ Intimate Mechanisms Nature of leaving & entering groups L5M-X W L5M + X For dissociative mechanism rate is dependent the leaving group i.e. M-X bond strength e- - donor σ-donor like NH3, H-, π-donor→ σ + filled π-orbital e.g. Br-, O-, Cp-, RO-, RN2 e--acceptor→ σ-donor + empty π* orbitals e.g. CO Ligands σ-donors H->PR3>SCN->I->CH3-,CO>CN->Br->Cl>NH3>OHπ-acceptors – CO~C2H4>CN->NO2->NCS->I->Br- Leaving group order for Oh compounds HCO3->> NO3-> I-~ H2O~ Br->Cl-~ SO42-> SCN->F->CH3COO->NCS>NO2->NH3>OH->CN- Size of the Coordination Sphere & Metal ION Large Coordination sphere favors associative mechanism Small Coordination sphere favors dissociative dtype pathway. e.g. 1st → Oh favors D-type 2nd → Oh may be A-type 3rd → Oh favors A-type Compare water exchange in [Rh(H2O)6]3+ & [Ir(H2O)6]3+ 1:3 Coordination Sphere Coordinatively saturated Favors D- type Coordinatively unsaturated Favors A-type Mechanism 1. Leaving group e.g. [Co(NH3)5X]2+ +H2O→ [Co(NH3)5(H2O)]3+ [Co(NH3)5(H2O]3++Y- → [Co(NH3)5(Y)]2+ Rate is dependent on leaving group and is independent on entering group. i.e. Co-X bond breaking is the rate determining step. Evidence? Aquation or acid hydrolysis Aquation or acid hydrolysis L5MXn+ + H2O→ [L5M(H2O)]n+1 +XAnation [L5M(H2O)]n+1 + Y- →L5MYn+ + H2O Rate constants for acid aquation of some Oh compounds of Co(III) of the type [Co(NH3)5(X)]2+ at 25 C Compound k (sec-1) [Co(NH3)5(OP(OCH3)3)]3+ [Co(NH3)5(NO3)]2+ [Co(NH3)5(I)]2+ [Co(NH3)5(H2O)]3+ [Co(NH3)5(Cl)]2+ [Co(NH3)5(SO4)]+ [Co(NH3)5(F)]2+ [Co(NH3)5(N3)]2+ [Co(NH3)5(NCS)]2+ 2.5x10-4 2.7x10-5 8.3x10-6 5.8x10-6 1.7x10-6 1.2x10-6 8.6x10-8 2.1x10-9 5.0x10-10 Note: k depends heavily On leaving group X ∴Confirm Dissociative pathway Limiting Rate constants for anation by Yn- and water exchange (ke) at 45 C [Co(NH3)5(H2O)]3+ Yn- k (sec-1) k/ke H2O N3SO42ClNCSH2O H2PO4- 100x10-6 100x10-6 24x10-6 21x10-6 16x10-6 5.8x10-6 7.7x10-7 1.0 0.24 0.21 0.16 0.13 Rate is insensitive to entering group i.e. evidence support Dactivation Further evidence come from the substitution studies of Yby XWhere no direct substitution was Observed. Example [Ru(edta)(H2O)]- + L → [Ru(edta)L] [Ru(edta)L] + H2O→ [Ru(edta)(H2O)]+ L Rate constants for substitution by L(kl) of [Ru(edta)(H2O)]- and for aquation kaq of [Ru(edta)(H2O)]-at 25ºC L Pyridine NCSCH3CN kL(M-1sec-1) 6.3x103 2.7x102 3.0x10 kaq(sec-1) 0.061 0.5 3.2 Ligands COOCOO- -OOC N N N COO- pyridine edta Steric Effect of Inert Ligands Crowding around the metal ion will retard the rate of reactions that occur by an a-type mechanism and to speed up those occurring via a d-type mechanism. Steric Effect N H H N N H H N CH3 H3C N H H N CH3 N H H N H3C CH3 H3C H3C N H H N H3C CH3 N H H N CH3 Electronic Effect of Inert Ligands N N - Co - L N N + + N π-donation by a cis ligand into the p component of an empty d2sp3 hybrid. L N - Co N N π-otbital of trans ligand is ┴ to d2sp3 hybrid. + Rearrangement N N - Co + N + N - L π-donation from trans Ligand can occur upon rearrangement to Trogonal bipyramidal Activation Parameters ∆H‡ Enthalpy of activation is a measure of the height of the energy barrier, particularly bond strengths within and between reactants, which must be overcome to attain the transition state; If ∆H# <D(M-X), then bond is broken during activation wher D = dissociation energy. ∆S‡ relates to probability of reaction-it includes contributions from steric bulkand orientation of reactants ∆S‡ ~ 0 for unimolecular reactions + for dissociative The more negative the value of ∆S‡ the lower the reaction rate. Activation Parameters logk, sec-1 k α ∆G0; more thermodynamically favored reactions proceed faster Hamond’s postulate: Two consecutively occurring states that have similar energy along a reaction coordinate will involve only small structural reorganization. ∆G0 = -RTlnK logK Slope = 1 for dissociative; i.e. TS or intermediate falls late in the reaction coordinate; i.e. product-like Slope ~ 0.5 for associative, TS or [x] falls early in the reaction coordinate; i.e. reactants-like. Volume of activation + dissociative & - associative Activation Parameters, The Eyring Equation k = RT/Nhe∆S‡/Re-∆H‡/RT kNh/RT = e∆S‡/Re-∆H‡/RT keff = e∆S‡/Re-∆H‡/RT Where N is Avogadro’s Numbern= 6.02x1023 mol-1 H = Plank’s constant = 6.6x10-34 J.S. lnkeff Slope = -∆ ∆H‡/R Note : ∆H‡ = Ea -RT -1 1/T, K Ligand Substitution Reactions on Square Planar Complexes d8 metal ions such as [AuIII, NiII, PdII, PtII, RhI and IrI ] usually form four coordinate square planar compounds, especially with strong ligands. X L M L ML3X + Y → ML3Y + X L Rate = k1[ML3X] + k2[Y][ML3X] Mechanism for Sq. Substitution L L M L X +Y L Y Y L M M L L X L L L L L M L X Y L M L X tbp Trigonal bipyramidal may be an intermediate for A type of TS for Ia 5-coordinate species may be isolated as in [Ni(CN)5]3- and can be isolated as salt of [Co(en)3]3+ Reactivity order NiII>PdII>PtII Y Nucleophilicity order for square planar compounds: CN-~CO>PR3>H->I->Cl->H2O Leaving group order H2O>Cl->I->H->PR3>CO~CN- Rate Law Rate = (k1 + k2[Y])[ML3X] Indicate two parallel reaction paths. In the presence of a large excess of the entering ligand [Y] remains constant throughout the reaction Rate = -d[ML3X]/dt = kobs.[ML3X] where kobs. =k1 +k2[Y] A plot of kobs. versus [Y], Straight line with k1 = intercept and k2 is the nucleophilicity strength of Y. Rates of reaction of SCN- kobs = k1 + k2[Y] kobs, sec-1 I- C6H5SH Br-NH2OH N3NO2- [Y] Rate of reaction of trans-[Pt(py)2Cl2 in methanol @ 30 C as a function of the concentration of entering ligand. Rate Law Note all ligands have the same intercept, k1, implying that they go through the same species. k2 is different from one ligand to another. Since the only species present in solution in addition to the ligands is the solvent molecules this implies that the solvent is involved in the first term and the second term in the ligand participation Summary Two pathways are involved: (1) first order in complex (k1), does not involve Y in rate determining step. This mechanistic pathway is solvent mediated. (2) first order in complex and Y (k2), Y involved in rate determining step Mechanism L L K1, slow L Pt X +S L Pt X L YL L Pathway #1 S Pt L L fast Y Pathway 2 L L k2, slow L Pt X +Y Y L Pt X L XL L Pt L L fast Y Summary Both reactions proceed via an associative process (A), involving a trigonal bipyramid transition state. Is there any chemical justification for this transition state? (i) Many five coordinate transition metal complexes are known, e.g., Fe(CO)5, [CoL2(CO)3] + , [Ni(CN)5]3(ii) ML3X complexes are sterically and electronically unsaturated and have space for Y to coordinate. Evidence Rate law is consistent with associative mechanism Charge on the metal center-has no effect on the rate of reaction Steric effect-significant increase in rate was observed for less hindered ligands, trans- is faster than cis C6H5>2Me-C6H4>2,4,6-Me3-C6H2Note trans>cis For the substitution reaction Pt(PEt3)2RCl + Y- → Pt(PEt3)2RY + Cl- Rate law ML3X S = solvent Y S ML3Y Y ML3S Schematic representation of parallel reactions Effect of Entering & Leaving Ligands on Rate Rate constants for ligand displacement in some square planar Pt(II) compounds Trans-[Pt(py)2Cl2 + Y → trans[Pt(py)2YCl] + ClY TC k2(M-1sec-1) nPt CH3OH CH3OClNH3 N 3ICNPPh3 25 25 30 30 30 30 25 25 2.7x10-7 very slow 4.5x10-4 4.7x10-4 1.55x10-3 1.07x10-1 4.00 249 00 <2.4 3.04 3.07 3.58 5.46 7.14 8.93 Effect of Entering & Leaving Ligands on Rate Rate constants for ligand displacement in some square planar Pt(II) compounds trans-[Pt(dien)X]+ + py → trans[Pt(dien)(py)]2+ + X X kobs(M-1sec-1) CN- SCN IClH2O NO3- 1.7x10-8 3.0x10-7 1.0x10-5 3.5x10-5 1.9x10-3 very fast Factors affecting rate of substitution 1. Nucleophilicity PtCl2Py2 + Y - → PtClYPy2 + ClStandard Reaction, and methanol as a standard nucleophile, a base with unshared pair of electrons such as NH3, I-, Cl-, R-, PR3 nPt = logk2(Y)/k1(CH3OH) = logk2(Y) – logk1(CH3OH) Factors Affect Nucleophilicity Basicity Oxidation potential of Y Nucleophilicity order snPt = logk2(Y)/k2(solvent) S = nucleophilicity discriminating factor I - > Br - > Cl - >> F PR3 > AsR3 > SbR3 >> NR3 Sulphur > Oxygen Soft bases are better nucleophiles for PtII as it is a soft acid Trans –ligands & trans effect "The trans effect is defined as the effect of a coordinated ligand upon the rate of substitution of ligands opposite to it.“ For Pt II compounds the order of trans effect H2O ~ OH - ~ NH3 ~ NR3 < Cl - ~ Br - < SCN ~ I - ~ NO2 - ~ C6H5 - < CH3 - < PR3 ~ AsR3 ~ H - < olefins ~ CO ~ CN- NH3 NH3 NH3 Pt Cl- NH3 NH3 NH3 Pt Cl -- NH3 Cl Pt Cl Cl- NH3 - NH3 Trans effect in synthesis Cl NH3 NH3 Cl Pt Cl Cl Pt Cl Cl NH3 NH3 Pt Cl NH3 Cl Cl Try Synthesis of cis and trans- isomers of [PtCl2(NH3)(PPh3)] given the reactants PPh3, NH3 and [PtCl4]2- Origin of the trans-effect Electronic (Molecular Orbitals) σ-donor ligands such as H-, R-, increase electron density around the metal. π-acceptor ligands such as CO, CN-, PR3, pback bonding weaken the bond trans to it. Trans-effect T.S. Tpy σ-donor π-acceptor PR3 PR3 M T Ground state X M Steric Effect of Inert Ligands Steric effect is unclear Trans-Pt(PEt3)2RCl] + y → trans-[Pt(PEt3)2RY] + ClR C6H5 C6H5 o-C6H4CH3 o-C6H4CH3 2,6-C6H3(CH3)2 2,6-C6H3(CH3)2 Solvent methanol DMSO methanol dmso methanol dmso Y=CN 3.61 very fast 0.234 3.54 8.49x10-3 3.17x10-2 Y =SC(NH2)2 3.30 0.532 0.652 0.106 4.94x10-2 9.52x10-3 Steric Effect Steric effect is significant, hints to dissociative pathway, i.e. although associative pathway is feasible but dissociative pathway may become operative in the presence of bulky Ligands. Conclusion (i) 1 st order in substrate and nucleophile, (ii) k1 (solvent) and k2 (entering ligand)- solvent is important A number of factors determine reactivity (i) Nucleophilicity - different from basicity (ii) cis and trans effects - kinetic effects, not the same as trans influence - thermodynamic effect (iii) Leaving group effect (iv) Rate depends on the metal M Predictive synthesis. Redox Reactions Electron-transfer A + B- → A- + B 1. Inner sphere Transition state involve interpenetrationg coordination sphere or bridging ligands or interaction between reactants which is common to both coordination sphere & serves as channel through which electron flows. Taube & Meyer Cr2+ & Cr3+ Co(H2O)62+ (Labile) & [Cr(NH3)5Cl]2+ (innert) Co(H2O)62+ 5NH4+ + H+ 2+ [Cr(NH3)5Cl] → [Co(H2O)6]2+ + [ClCr(H2O)6]2+ + Inner sphere [Co(NH3)5Cl]2+ + Cr2+ W [Co(NH3)5ClCr]4+ [Co(NH3)5ClCr]4+ → CrCl2+ + [Co(NH3)5]2+ [Co(NH3)5]2+ + 5H+ W Co2+ + 5NH4+ Cr3+ inert to substitution with k = 2.9x10-8 M-1sec-1 for Cl- anation of Cr3+ For the reduction k = 6x105 M-1sec-1 ∴ Cr-Cl could not have been formed from substitution of free Cl- Outer-sphere Electron Transfer The interaction between the oxidant and reductant at the time of electron transfer is small, therefore the coordination shells are intact; i.e. through space , diffusion control A e- B- Example Outer-sphere Electron Transfer [Fe(me2bipy) 3]2+ + [Fe(bipy) 3]3+ W [Fe(me2bipy) 3]3+ + [Fe(bipy) 3]2+ Rate = k [Fe(me2bipy) 3]2+ [Fe(bipy) 3]3+ = 108 M-1sec-1 Rate of substitution is ~ 104 M-1sec-1 For outer sphere the redox rate must be faster than the substitution rate.