The trans effect in cobalt(III) complexes. Kinetics and mechanism of substitution of cobalt(III) sulphito complexes

Abstract

Equilibrium constants K and rate constants kf have been measured, at 25°C and ionic strength of 1.0, for the substitution of the labile water molecule in trans-[aquabis(ethylenediamine)sulphito-cobalt(III)] ion by thiosulphate ion (K = 1.8×102 mol-1 1., kf = 1.27×103 mol-1 1. s-1), thiocyanate ion (2.5×103, 2.75×102), nitrite ion (1.0×103, 2.06×102), azide ion (2.9×102, 2.4×102) ferricyanide ion (-, 1.72×103), hydrogen azide (< 1.2,1.4×10), ammonia (3.0, 6.7) and imidazole (2.6×102, 5.2). ��� The correlation of these rate constants with charge on the incoming ligand, as well as a decrease in the apparent second-order rate constants observed at high concentrations of the anionic ligands, requires a rapid outer-sphere pre-equilibrium step followed by a rate- determining dissociative interchange of the incoming ligand with the bound water molecule. The activation energy of the thiocyanate substitution was found to be 48 kJ mol-1. Aquation of cis- [azidobis(ethylenediamine)-sulphitocobalt(III)] ion, in the range of hydrogen ion concentration between 10-2 and 0.2 M, was found to give the trans-aquasulphito complex with a first-order rate constant consistent with the equation ��������������������������� k = 4.9×10-4[H+]+1.0×10-5 s-1 at 25°C and ionic strength 1.0.