Reversible non-complementary redox reactions involving platinum complexes

Abstract

The kinetics and equilibria of the reversible system (i)[L = NH₃, NMeH₂, NEtH₂, NPrH₂, or ½en (en = ethylene-[PtL₄]²⁺+ 2Fe³⁺+ 2Br^–⇌trans-[PtL₄Br₂]²⁺+ 2Fe²⁺(i) diamine)] have been studied in aqueous 0.50 mol dm^–3 perchloric acid at I= 1.0 mol dm^–3. The pseudo-first-order rate constants have the form k_obs.=k_f[Fe³⁺][Br^–]+k_f′[Fe³⁺][Br^–]² for the oxidation of platinum(II) complexes, and k_obs.=k_r[Fe²⁺] for the reduction of platinum(IV) derivatives. The rate of oxidation of the platinum(II) complexes by Fe^III is decreased by the presence of Fe^II. The results are consistent with a redox mechanism involving two successive one-electron-transfer steps. Changes of ligands are found to influence the kinetic and thermodynamic parameters in the same direction, the oxidation of the platinum(II) complexes being kinetically and thermodynamically favoured in the order NPrH₂ < NEtH₂ < NMeH₂ < NH₃ < en. The steric hindrance of the amines is considered to be the factor responsible for the observed trends. The kinetic and thermodynamic parameters are linearly correlated with the steric constants of the amines.