User:Benjah-bmm27/degree/2/FCM

Transition metals 2, FCM

"Co-ordination Chemistry of the Transition Metals, Part 2"

Relevant textbooks

• Richard A. Henderson, The Mechanisms of Reactions at Transition Metal Sites (Oxford Chemistry Primer) (1993) (on Amazon.co.uk)

Investigating mechanisms

Temperature dependence of rate constants

• Over fairly narrow range of temps, rate constant varies according to Eyring equation:
  • $k_{R}=\left({\frac {k_{\mathrm {B} }T}{h}}\right)\mathrm {exp} \left({\frac {\Delta S^{\ddagger }}{R}}\right)\mathrm {exp} \left(-{\frac {\Delta H^{\ddagger }}{RT}}\right)$,
    • where:
    • $\ k_{R}$ = rate coefficient
    • $\ T$ = temperature in kelvin
    • $\ \Delta H^{\ddagger }$ = enthalpy of activation
    • $\ R$ = gas constant
    • $\ k_{\mathrm {B} }$ = Boltzmann constant
    • $\ h$ = Planck's constant
    • $\ \Delta S^{\ddagger }$ = entropy of activation

Ligand substitution reactions

• Associative, dissociative and interchange reaction mechanisms: Dissociative substitution, Associative substitution, Sn1CB mechanism

Effect of trans ligands

• Trans series: F⁻ < OH⁻ < H₂O < NH₃ ≈ py < Cl⁻ < Br⁻ < I⁻ ≈ SCN⁻ ≈ NO₂⁻ < CH₃⁻ < H⁻ ≈ PR₃ < CN⁻ ≈ CO

• Trans influence (AKA thermodynamic trans effect): good σ donor trans ligand puts electron density on Pt, weakening and lengthening Pt-X bond in ground state

• Trans effect (AKA kinetic trans effect): good π acceptor trans ligand stabilises five coordinate trigonal bipyramidal intermediate (removes electron density from 18e Pt)

Substitution in octahedral complexes

• Rate depends strongly of leaving group X but weakly or not at all on entering group Y (so D or I_d mechanism)

• at high [Y], rate = k[ML₅X] (suggesting D mechanism)

• at low [Y], rate = k[ML₅X][Y] (suggesting A mechanism)

• explained by Eigen-Wilkins mechanism (de:Eigen-Wilkins-Mechanismus, Manfred Eigen and R. G. Wilkins)

1. ML₅X and Y form a weakly-bound encounter complex, [ML₅X···Y] (which forms and dissociates rapidly in a so-called pre-equilibrium with eqm. constant K_o)
2. The encounter complex [ML₅X···Y] rearranges to [ML₅Y···X], i.e. breaking the M-X bond and forming an M-Y bond
3. The rearranged complex [ML₅Y···X] dissociates to the products ML₅Y and X

step	equation	symbol	rate
1	ML5X + Y ⇌ [ML5X···Y]	Ko	fast
2	[ML5X···Y] ⇌ [ML5Y···X]	k	slow (RDS)
3	[ML5Y···X] ⇌ ML5Y + X	?	fast

${\text{rate}}={\frac {kK_{\text{o}}\left[{{\text{ML}}_{\text{5}}{\text{X}}}\right]_{\text{o}}\left[{\text{Y}}\right]_{\text{o}}}{{\text{1}}+K_{\text{o}}\left[{\text{Y}}\right]_{\text{o}}}}$

Electron transfer

• Electron transfer: Oxford notes

Inner sphere ET

• Inner sphere electron transfer: Housecroft p. 895, ET via a covalent linkage (a bridging ligand), IUPAC definition, Henry Taube, Creutz-Taube Complex

Outer sphere ET

• Outer sphere electron transfer
• Marcus theory:

Free energy of activation for self-exchange reactions:

$\Delta G_{xx}^{\ddagger }=\Delta _{\text{w}}G^{\ddagger }+\Delta _{\text{o}}G^{\ddagger }+\Delta _{\text{s}}G^{\ddagger }+RT\ln \left({\frac {k_{\text{B}}T}{hZ}}\right)$

Rate of self exchange reactions:

$k_{xx}=\kappa Ze^{-{\frac {\Delta G^{\ddagger }}{RT}}}$

where Z ≈ 10¹¹ dm³ mol⁻¹ s⁻¹ and the transmission coefficient κ ≈ 1

• Marcus equation:

$k_{12}=\left({k_{11}k_{22}K_{12}f_{12}}\right)^{\tfrac {1}{2}}$

$\log f_{12}={\frac {\left({\log K_{12}}\right)}{4\log \left({\frac {k_{11}k_{22}}{Z^{2}}}\right)}}$

$\ln K_{12}={\frac {nFE_{\text{cell}}^{\circ }}{RT}}$