Kinetics and mechanism of the thermal decomposition of the thiosulphatopentaamminecobalt(III) ion in dilute aqueous acid

Abstract

In acetic acid – sodium acetate buffer of pH 5.6 (25 °C) the Co(NH3)5S2O3+ ion undergoes redox decomposition rather than aquation. First-order kinetic are observed and the reaction products Co2+, NH3, and S4O62− are due to internal reduction of Co(III) by coordinated S2O32−. In dilute perchloric acid of pH < 4 the rate is retarded, first-order plots are not linear, and S4O62− is not a major product of the reaction. It is proposed that, in dilute HClO4, protonation of Co(NH3)5S2O3+ depletes the concentration of the reactive complex and that decomposition of coordinated HS2O3− occurs. Conversion of O-bonded S2O32− to S-bonded S2O32− in the reactive complex is also considered.