Studies in nickel(IV) chemistry. Part 4. Kinetics of the electron transfer from L-ascorbic acid to tris(dimethylglyoximato)nickelate(IV) in aqueous medium

Abstract

The kinetics of electron transfer from L-ascorbic acid (H₂A) to tris(dimethylglyoximato)nickelate(IV), [Ni(dmg)₃]^2–, follow a pseudo-first-order disappearance of the Ni^IVcomplex in aqueous medium at 35 °C and I= 0.25 mol dm^–3. The pseudo-first-order rate constant (k_obs.) conforms to k_obs.=k_ad+k_red, where k_ad refers to the pseudo-first-order rate constant for the independently determined proton-assisted decomposition of the Ni^IV complex and K_red, to that for the intermolecular reduction of Ni^IV by H₂A. The rate constants for the proton-assisted decomposition range from 4.6 to 2.6 × 10^–7 s^–1(pH range 1.6–12.3) and those for the reduction reaction range from 5.1 × 10² to 8 dm³ mol^–1 s^–1(pH range 1.6–11.1). The results are interpreted in terms of a probable mechanism involving a rate-determining outer-sphere one-electron transfer from the reductant to the oxidant Ni^IVspecies present in the solution, followed by a subsequent and kineticaliy indistinguishable rapid reaction of the Ni^III intermediate.