Redox reaction mechanisms in non-complementary processes. Part II. Kinetics of platinum(II)–iron(III) and iron(II)–platinum(IV) interconversions

Abstract

The kinetics of the equilibration reactions (i)(L = NH₃, NH₂Me, NH₂Et, NH₂Pr, or ½en; X = Cl or Br) have been cis- or trans-[PtL₂X₂]+ 2Fe³⁺+ 2X^–⇌cis- or trans-[PtL₂X₄]+ 2Fe²⁺(i) studied in water in the presence of 0·5M-perchloric acid at 1M ionic strength. The rate law for the reduction of the platinum(IV) complexes has the form : rate =k_r[Pt^IV][Fe²⁺]. The rate law for the oxidation of the platinum(II) bromo-complexes, when no iron(II) is added to the reacting mixture, is : rate =k_f[Pt^II][Fe³⁺][Br^–]+k_f′[Pt^II][Fe³⁺][Br^–]². When an excess of iron(II) is added to the reacting mixtures the reactions do not go to completion and the rates of approach to equilibrium strongly depend on the amount of iron(II) added. The forms of the rate laws are consistent with a mechanism involving a platinum(III) intermediate and two one-electron redox steps. A five-co-ordinate platinum(II) complex, [PtL₂Br₃]^–, is postulated to be responsible for the k_f′ oxidation path. The free energies of activation for both oxidation of platinum(II) and reduction of platinum(IV) complexes are found to parallel the overall free-energy changes of the reactions. A change of the geometric form of the complexes has no effect on the reaction rate or the equlibrium constant of the redox reaction. trans-[PtL₂Br₄] is 5–15 times less reactive than the corresponding chloro-complexes, mainly as a consequence of the formation of a more stable oxidation product, FeX²⁺, when X is Cl rather than Br.