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, k_obs=(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 k_obs 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.