Studies in nickel(IV) chemistry. Part 6. Kinetics of electron transfer from phenylhydrazine to tris-(dimethylglyoximato) nickelate(IV) in aqueous medium

The kinetics of electron transfer from phenylhydrazine(S) to tris-(dimethylglyoximato) nickelate(IV), Ni(dmg) (dmg^2? = dimethylglyoximate dianion), have been studied in aqueous medium in the range of 6.21 ? pH ? 12.2. The kinetics exhibit a pseudo-first-order disappearance of Ni(dmg) when excess S is present. The pseudo-first-order rate constants k_obs are almost linearly dependent on S]₀ for varying concentrations of the reductant. At constant S]₀, the k_obs?pH profile is U shaped. The k_obs values register a decrease as the H⁺] is increased in the pH range of ∽12.2–9.5, remain almost constant (minimum) in the range of ∽9–8, and then again linearly increase as H⁺] is increased in the pH range of ∽7–6.21. Results are interpreted in terms of a probable mechanism involving outer-sphere electron transfer from the phenylhydrazine and phenylhydrazinium cation species to the unprotonated and one-protonated species of the Ni(IV) complex. The reduction rate appears to be dependent on the nature of the species (unprotonated and one-protonated) of the oxidant Ni(IV) complex. The phenylhydrazinium cation reduces the Ni(IV) complex at least one order of magnitude faster than does the neutral reductant species. The major product of the oxidation of phenylhydrazine by the Ni(IV) complex is 4-hydroxyazobenzene with a small amount of phenol.