σ-Bonded organotransition-metal ions. Part IV. Kinetics and mechanism of the acid-catalysed decomposition of the 4-pyridiomethylpentacyanocobaltate(III) ion

Abstract

Potassium 4-pyridylmethylpentacyanocobaltate(III) monohydrate, prepared by the reaction of cobaltous cyanide with 4-bromomethylpyridinium bromide, is stable to air in aqueous solution. In solutions of pH 9·2 and below the pyridine ring is protonated to form the 4-pyridiomethylpentacyanocobaltate(III) ion and in perchloric acid of ca. 4M a second proton is added, presumably to one of the cyanide ligands. The kinetics of decomposition of these species have been investigated. In > 1M-perchloric acid the rate of decomposition increases linearly with increasing acid concentration to a maximum in 7·5M-acid, owing to the unimolecular decomposition of the protonated 4-pyridiomethylpentacyanocobalate(III) ion to the 4-cyanomethylpyridinium ion. At lower acidities (pH 1–5) the kinetics are complicated by the slowly established equilibrium between the 4-pyridiomethylpentacyanocobaltate(III) ion and the 4-pyridiomethylaquotetracyanocobaltate(III) ion and by the further decomposition of the latter ion at a rate comparable with the initial rate of approach to equilibrium. The kinetics in this pH region can however be simplified by the addition of small quantities of HCN sufficient to suppress the formation of the aquotetracyano-complex so that only the acid-catalysed decomposition of the pentacyano-complex is observed.