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, kobs, 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 kobs=k[H+]+kI(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.