Kinetics and mechanisms of reduction of a nickel(IV) complex by ascorbate ion

Abstract

The kinetics of reduction of a nickel(IV) oxime complex by ascorbate ion to give the corresponding nickel(II) species are reported. Ascorbate ion is the sole reductant with second-order rate constants 3.02 × 10⁵ dm³ mol^–1 s^–1 and 1.36 × 10⁴ dm³ mol^–1 s^–1 for reactions with protonated and unprotonated forms of the oxidant respectively at 25.0 °C and 0.10 mol dm^–3 ionic strength. The origin of the protonation of nickel(IV), pK_h 3.70, is discussed. Electron spin resonance experiments suggest that the redox mechanism involves two discrete one-electron transfer steps with formation of a nickel(III)–oxime intermediate. The first step in the reduction goes by an outer-sphere pathway and a self-exchange rate around 10⁴ dm³ mol^–1 s^–1 is calculated for the Ni^IV–Ni^III couple. Studies with optically active solutions of nickel(IV) show no chiral discrimination in the reaction.