Redox reaction mechanisms in non-complementary processes. Part I. Redox reactions between the tetrabromobismethylaminoplatinum(IV)–dibromobismethylaminoplatinum(II) and iron(III)–iron(II) systems in the presence of bromide ions

Abstract

The mechanism of the reversible redox reaction (i) has been studied in the presence of 0·5M-perchloric acid at trans-[Pt(NH₂Me)₂Br₂]+ 2Fe³⁺+ 2Br^–⇌trans-[Pt(NH₂Me)₂Br₄]+ 2Fe²⁺(i) 1M ionic strength. The rate law for the reduction of trans-[Pt(NH₂Me)₂Br₄] by iron(II) in the presence of bromide is: rate =k_r[Pt^IV][Fe²⁺]. The rate law for the oxidation of trans-[Pt(NH₂Me)₂Br₂] by iron(III) in the presence of bromide, when no iron(II) is added to the reacting mixture, is: rate =k_f[Pt^II][Fe³⁺][Br^–]. The rate of oxidation of the platinum(II) complex is lowered by an excess of iron(II) in the reacting mixture. Moreover, in these conditions the rate is not strictly linearly dependent on the bromide concentration. In such cases the pseudo-first-order rate constant of approach to equilibrium at 50°C, when iron(III), iron(II), and bromide are all present in the reacting mixture in concentrations considerably higher than those of the platinum complexes, is K_obs/S^–1=4·1[Fe³⁺][Br^–]//_{1 + 1·25 × 10^–2[Fe²⁺]//[Fe³⁺][Br^–]}+4·1 × 10^–3[Fe²⁺]//_{1 + 80 [Fe³²⁺][Br^–]//[Fe²⁺]}(ii) as shown in equation (ii). The form of the rate constant is quantitatively consistent with a mechanism involving a platinum(III) intermediate and two, one-electron redox steps. Values for the equilibrium constant of the reaction calculated from the concentrations of the various species present at equilibrium (6·2 × 10⁴ l² mol^–2 at 50°C), and from the specific rate constants derived on the basis of the suggested mechanism (8 × 10⁴ l² mol^–2 at 50 °C) agree reasonably well and provide additional confirmation for the suggested mechanism.