Kinetics and mechanism of substitution in trans-dichlorobis(ethylenediamine)-cobalt(III) chloride by organic bases in non-aqueous solvents

Abstract

Kinetics of substitution of a chloride in trans- dichlorobis(ethylenediamine)cobalt(III) cation by five organic bases (piperidine, diethylamine, cyclohexylamine, butylamine and benzylamine) in methanol, dimethylformamide and mixtures of the two solvents are reported. Rates are high in methanol but appreciably smaller in dimethylformamide and increase with the increase in methanol content in the mixtures. A retardation was observed by the addition of amine hydrochloride to the methanolic medium. These observations have been explained on the basis of catalysis by methoxide ion and a mechanism (S_N1CB) in terms of a rapidly established acid-base equilibrium resulting in a conjugate base, which dissociates in the rate- determining step, has been proposed. Activation parameters have been evaluated on the basis of the rate data at four different temperatures.