Kinetics and mechanism of the electron-transfer reaction between chloropenta-amminechromium(III) ion and chromium(II) in acetate medium

Abstract

The Cr^II-catalysed decomposition of chloropenta-amminechromium(III) ion in an acetate buffer medium obeys the rate law d[Cr(OAc)₃]/dt=k₀[Cr(NH₃)₅Cl²⁺][Cr^II]^½_T[OAc^–]⁰, where k₀= 0·58 ± 0·01 M^–½ min.^–1 at 25·0° and unit ionic strength (NaClO₄). The corresponding activation energy is 14·2 kcal./mole. Evidence is presented for a reaction mechanism involving an unstable monomeric chromium(II) complex, probably Cr(OAc)₃^–. The second-order rate constant for the reaction Cr(OAc)₃^–+ Cr(NH₃)₅Cl²⁺ is approximately 70M^–1 min.^–1 under the conditions stated. The predominant dinuclear chromium(II)–acetate complex does not participate in the electron-transfer reaction.