Kinetics of the oxidation of chloride ions by tris(2,2′-bipyridine)nickel(III) ions in aqueous perchlorate media

Abstract

The kinetics of oxidation of chloride ions by the tris(2,2′-bipyridine)nickel(III) ion, [Ni(bipy)₃]³⁺, have been investigated in aqueous media. Using conventional spectrophotometry, the orders in the reactant concentrations were found to be variable at an ionic strength of 5.00 mol dm^–3. However, using stopped-flow spectrophotometry to investigate the oxidation at an ionic strength of 2.50 mol dm^–3 at high [Cl^–], the order in [Ni^III] was always unity and plots of the pseudo-first-order rate constants ×[Cl^–]^–1 with [Cl^–] at constant acidity were linear with an intercept on the ordinate, and invariant with changes in the acidity. This is interpreted in terms of the involvement of outer-sphere complexes of Cl^– and [Ni(bipy)₃]³⁺ in the mechanism in two parallel pathways. The enthalpies and entropies of activation are compared with those for the oxidation of other substrates by [Ni(bipy)₃]³⁺.