Preparation and characterisation of cis- and trans-tetra-amminebis(sulphito)cobaltate(III) anions and the kinetics of redox decomposition aqueous acid solution

Abstract

In aqueous sulphite solution, trans-[Co(NH₃)₄(H₂O)(SO₃)]⁺ rapidly forms the bis(sulphito) complex which has been isolated as an unstable yellow solid, trans-Na[Co(NH₃)₄(SO₃)₂],2H₂O. The cis-isomer has been prepared as the brown salts cis-Na[Co(NH₃)₄(SO₃)₂],2H₂O and cis-NH₄[Co(NH₃)₄(SO₃)₂],3H₂O. I.r. and u.v.–visible spectra are reported. The latter are in disagreement with literature spectra and cis–trans-assignments based on them are reversed. In perchloric acid solution, aquation of ligands trans to sulphite results in rapid formation of trans-[Co(NH₃)₄(H₂O)(SO₃)]⁺ and [Co(NH₃)₂(H₂O)₂(SO₃)₂]^– from the trans- and cis-isomers respectively. Rate constants, k_obs, for redox decomposition of the cis-isomer under air-free conditions are of the form (i) at [H⁺]= 0·02–0·20M; at 20·0 °C and I= 1·00M(LiClO₄), k=(1·39 ± 0·02)× 10^–2 l mol^–1 s^–1, ΔH^‡= 26·5 k_obs=k[H⁺]+k_I(i)± 0·4 kcal mol^–1, and ΔS^‡= 23·4 ± 1·3 cal K^–1 mol^–1. The acid-independent term contributes only ca. 3% of the observed rate at 0·1M-HClO₄.