Redox decomposition of trans-tetra-ammineaquosulphitocobalt(III) in aqueous solution
Abstract
The complex trans-[Co(NH3)4(H2O)(SO3)]+ decomposes to Co2+ in aqueous acid solution. Kinetic plots exhibit an initial period of curvature followed by a first-order process, kobs=(1·14 ± 0·03)× 10–4 s–1 at 25·0 °C, ΔH‡= 26·5 ± 0·1 kcal mol–1, and ΔS‡= 12·5 ± 0·4 cal K–1 mol–1[I= 1·00M(LiClO4)]. The rate of reaction is independent of [H+] in the range 0·01–1·00M. A mechanism is proposed relating kobs to a rate-determining isomerisation to the cis-complex, followed by rapid conversion to the complex [Co(NH3)3(H2O)2(SO3)]+ which probably undergoes an internal redox reaction. Somewhat higher rates have been observed in the presence of molecular oxygen and evidence is presented for formation of the free radical HSO3.