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

Abstract

Equilibrium constants K and rate constants k_f 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., k_f = 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.