Octahedral cobalt(III) complexes in dipolar aprotic solvents. IV. Solvolysis and isomerization of dibromobisethylenediaminecobalt(III) ions inanhydrous NN-dimethylformamide and NN-dimethylacetamide

Abstract

The equilibrium system (SOL)+Br^-+cis-[CoBr₂ em]⁺ = cis-[CoBr(SOL) en₂]²⁺+2Br^-   =trans[CoBr₂en₂]²⁺+Br^-+SOL where SOL represents either of the solvents NN-dimethylformamide (DMF) or NN-dimethylacetamide (DMA), has been studied. The compounds cis- [CoBr(DMF) en₂](ClO₄)₂, and cis-[CoBr(DMA) en₂]XO₃,ClO₄, have been isolated and thus it has been possible to study these equilibria using as starting materials both the cis- and trans-dibromo complexes (cis- and trans-[CoBr₂ en₂]⁺) and the solvento complexes (cis-[CoBr(DXA) en₂]²⁺ and cis-[CoBr(DMF) en₂]²⁺). The mechanism of the bromide entry reactions cis-[CoBr(SOL) en₂]²⁺ +Br^- = cis- and trans-[CoBr₂ en₂]⁺+SOL is bimolecular in DMA while in DMF the reaction is dissociative: the rate showing bromide concentration dependence only in the concentration range where ion association is not complete. In both systems the isomerization proceeds mainly through the solvento complex; in dimethylacetamide there is evidence for a seven-coordinated intermediate. The isomerization equilibria are found to be bromide concentration dependent, cis-[CoBr₂ en₂]⁺ being favoured by high bromide concentrations. This is consistent with the lower stability of the trans-[CoBr₂ en₂]^- ion pair with bromide ion.