Kinetics and mechanism of reduction of a nickel(IV) complex by tris(1,10-phenanthroline)cobalt(II); spectrophotometric detection of a nickel(III) intermediate

Abstract

The kinetics of reduction of a nickel(IV) di(oxime–imine) complex, [Ni^IVL]²⁺(H₂L = 3,14-dimethyl-4,7,10,13-tetra-azahexadeca-3,13-diene-2,15-dione dioxime), by [Co(phen)₃]²⁺(phen = 1, 10-phenanthroline) are reported. The reaction is biphasic. An initial, rapid, pH-independent reaction between [Ni^IVL]²⁺ and [Co(phen)₃]²⁺ has a second-order rate constant of 3.2 × 10⁵ dm³ mol^–1 s^–1 at 25.0 °C and I= 0.10 mol dm^–3(Na[NO₃]) and leads to formation of a nickel(III) intermediate, [Ni^IIIL]⁺. Subsequent reductions of this intermediate and its protonated form [Ni^IIILH]²⁺ by [Co(phen)₃]²⁺ with second-order rate constants 8.3 × 10² and 4.1 × 10⁴ dm³ mol^–1 s^–1 respectively give the nickel(II) product. Both nickel(IV) and nickel(III) reactions proceed by outer-sphere electron transfer mechanisms. The nickel(III) intermediates are generated electrochemically and both forms are characterised. Data previously published for the ascorbate reduction of the nickel(IV) complex are reinterpreted.