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Transition metals 2, FCM[edit]

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

Relevant textbooks[edit]

Investigating mechanisms[edit]

Temperature dependence of rate constants[edit]

  • Over fairly narrow range of temps, rate constant varies according to Eyring equation:
    • ,

Ligand substitution reactions[edit]

Effect of trans ligands[edit]


  • 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[edit]

  • Rate depends strongly of leaving group X but weakly or not at all on entering group Y (so D or Id mechanism)
  • at high [Y], rate = k[ML5X] (suggesting D mechanism)
  • at low [Y], rate = k[ML5X][Y] (suggesting A mechanism)
  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 Ko)
  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

Electron transfer[edit]

Inner sphere ET[edit]

Outer sphere ET[edit]

Free energy of activation for self-exchange reactions:
Rate of self exchange reactions:
where Z ≈ 1011 dm3 mol−1 s−1 and the transmission coefficient κ ≈ 1