Studies in nickel(IV) Chemistry. Kinetics of the aquacopper(II)‐catalyzed oxidation of phenylhydrazine by tris(dimethylglyoximato)‐nickelate(IV) in aqueous medium

Abstract

AbstractThe kinetics of electron transfer in the redox system containing phenylhydrazine (S) and tris(dimethylglyoximato)nickelate(IV), in the presence of catalytic amounts of added Cu(II)aq, have been studied in aqueous medium at an ionic strength of 0.25M in the pH range of 6.01–9.06. The kinetics exhibit pseudo‐zero‐order disappearance of Ni(IV) when an excess of [S]0 and small amounts of Cu(II) are present. While the pseudo‐zero‐order rate constants are almost linearly dependent on [S]0 at constant [Cu(II)] and pH tending to become non‐linearly dependent on higher relative [S]0, they are linearly dependent on [Cu(II)] in a 20‐fold range. The pH‐rate profiles with low [S]0 and [Cu(II)] show a monotonic decrease in rates with increasing pH, the rates tending to attain limiting values at higher relative pH. Results are interpreted in terms of a probable mechanism involving the formation of precursor complexes of phenylhydrazine and Cu(II) species in the medium, followed by the rate‐determining breakdown of the precursors with concomitant electron transfer. The hydrolyzed species of Cu(II) reacts more slowly than does the aquacopper(II). Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The oxidation product of phenylhydrazine is 4‐hydroxyazobenzene.