The mechanism and kinetics of the substitution of azide, thiocyanate, and cyanide ions in aquopentacyanocobaltate(III) and pentacyanohydroxocobaltate(III)

Abstract

The substitution of pentacyanohydroxocobaltate(III), [Co(CN)₅(OH)]^3–, by azide ion is shown to take place by the reaction of aquopentacyanocobaltate(III), [Co(CN)₅(OH₂)]^2–, in equilibrium with it. Deviations from simple second-order rate laws for the substitution of azide, thiocynate, and cyanide into [Co(CN)₅(OH₂)]^2– at unit ionic strength and 40 °C are sufficiently small to be comparable with effects due to ionic activity variation and should not be accepted as evidence of a dissociative (D) mechanism. The reactions are reliably assigned to this class on the basis of activation volumes, solvent effects, and nucleophile competition experiments.