σ-Bonded organotransition-metal ions. Part II. Kinetics and mechanism of the aerobic acidolysis of penta-aquo(pyridiomethyl)chromium(III) ions

Abstract

The kinetics of the acidolysis of the three penta-aquo(pyridiomethyl)chromium(III) perchlorates have been studied in from 5·5M to 2 × 10^–3M perchloric acid, with and without added lithium perchlorate, in the presence of air. The uncatalysed reaction, which is evident in the range 0·2–0·002N-acid in the absence of added salt, involves the homolytic dissociation of the carbon–chromium bond, the reverse of which is largely prevented by the oxidation of chromous ion with air. The products and activation parameters are consistent with the formation of the pyridiomethyl radicals, and it is possible that a proportion of these radicals may also attack the pyridiomethylchromium ions. The acidolysis is catalysed by higher concentrations of perchloric acid or lithium perchlorate without changing the mechanism, but an even greater acceleration by sodium acetate is due to a change in mechanism. In all cases, in the presence of air, good first-order kinetics were observed.