Octahedral cobalt(III) complexes in dipolar aprotic solvents. I. The substitution of thiocyanate into the cis-Chlorodimethylsulphoxidobisethylenediaminecobalt(III) ion in anhydrous dimethylsulphoxide and NN-dimethylformamide

Abstract

The substitution of thiocyanate ion into the cis-chlorodimethylsulphoxidobisethylenediaminecobalt(III) ion, cis-(Coen₂(DMSO)Cl)²⁺, has been studied in the solvents dimethylsulphoxide (DMSO) and NN-dimethylformamide (DMF). In DMSO the reaction shows second-order characteristics which are accounted for by an ion-pair dissociative mechanism (S_NIIP). The activation energy is 30.1 kcal mole^-1. In DMF the entry is first-order, the rate determining step being solvolysis to the intermediate cis-(Coen₂(DMF)Cl)²⁺ which has been isolated as the nitrate. In high thiocyanate concentrations the rate shows some thiocyanate dependence due to the competition of ion-paired thiocyanate with the DMF solvent for coordination following the DMSO dissociation. The activation energy for this substitution is 17.5 kcal mole^-1.