Mechanism of octahedral substitution reactions in non-aqueous solutions. Part V. The chloride anation of cis- and trans-chloro-methanolobis(ethylenediamine)cobalt(III) cations in methanol

Abstract

The trans-[Co en₂CH₃O Cl]⁺ cation has been characterised as its perchlorate and tetraphenylborate. Addition of acid to its methanol solution immediately generates the labile trans-[Co en₂CH₃OH Cl]²⁺ cation which, in the absence of co-ordinating anions, isomerises to a mixture that contains mainly the cis-isomer. Addition of chloride to the solution of the trans complex leads to the formation of some trans-[Co en₂Cl₂]⁺ but does not reduce the rate of concurrent isomerisation. The rate of disappearance of trans-[Co en₂CH₃OH Cl]²⁺ is independent of chloride-ion concentration and so the fraction undergoing anation also does not change with chloride concentration. The rate of anation of the equilibrium mixture is much slower and depends upon the chloride concentration at low anion concentration but reaches a limiting maximum when the chloride concentration is high enough. In addition, the steric course of this anation depends on chloride concentration, the product consisting mainly of the trans-isomer at low halide concentration but containing >50% of cis-isomer at high chloride ion concentrations. It is suggested that this change in steric course arises from the reversibility of the isomerisation of the chloro-methanolocomplexes so that, at low chloride-ion concentrations a large portion of the equilibrium mixture anates by way of the more labile trans complex and thereby yields the trans-dichloro-product. The general behaviour is accounted for in terms of a pre-equilibrium association between the reagents followed by a dissociative “interchange” mechanism for the actual act of substitution.

Neither the reactivity of the chloro-methanolo-complexes nor the steric courses of their anation allow one to postulate their existence as necessary intermediates in the chloride exchange reactions of cis-[Co en₂Cl₂]⁺ in methanol. Consequently it is possible to rule out solvolysis, or solvent-assisted dissociation, as the rate-determining step of this reaction.