Mechanisms of reduction of a nickel(IV) oxime complex by 1,2- and 1,4-dihydroxybenzene compounds in aqueous perchlorate media
The kinetics of the reduction of a nickel(IV) oxime complex, [NiIVL]2+(H2L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by catechol, hydroquinone, and 2-methylhydroquinone (H2Q) have been investigated in aqueous perchlorate media in the pH range 3.5–6. Below pH 4.5 the reaction is monophasic with the NiIV–NiIII step rate-determining. Above pH 4.7 a biphasic process is observed with a rapid NiIV–NiIII reduction followed by a slower reaction of the nickel(III) intermediate. A complex pH dependence is attributed to reaction pathways involving several reductant and oxidant species related by proton equilibria. For the principal reaction pathways involving [NiIVL]2+ and HQ–, rate constants fall in the range ca. 107–108 dm3 mol–1 s–1. With H2Q as reductant, rate constants are ca. 105–106 times smaller. The specific rate parameters compare favourably with the rate constants determined by Marcus theory calculations.