Steric interaction of solvation and sterically enhanced halogeno complexation of manganese(II), cobalt(II) and nickel(II) ions in N,N-dimethylacetamide

The complexation of manganese(II), cobalt(II) and nikel(II) with bromide ions has been studied in N,N-dimethylacetamide(DMA) by calorimetry and spectrophotometry. The formation of [MBr]⁺, [MBr₂] and [MBr₃]^- (M=Mn, Co, Ni) was revealed in all the metal systems. Interestingly, the complexation is significantly enhanced in DMA over N,N-dimethylformamide (DMF). This is unusual because physicochemical properties of DMA and DMF as solvent are similar. Furthermore, extracted electronic spectra of individual complexes of Ni^II suggested the presence of a geometry equilibrium, [NiBr(DMA)₅]⁺=[NiBr(DMA)₄]⁺+ DMA, in DMA. A similar geometry equilibrium is also suggested, [NiBr₂(DMA)₃]=[NiBr₂(DMA)₂]+DMA. Such geometry equilibria were not observed in DMF. With regard to cobalt(II), electronic spectra show the presence of the four-coordinated [CoBr(DMA)₃]⁺ complex in DMA, unlike the six-coordinated [CoBr(DMF)₅]⁺ one in DMF. These facts suggest that a specific strong steric interaction operates between coordinating solvent molecules, which plays a key role in the complexation behavior of the divalent transition metal ions in DMA.