Sterically controlled complexation of manganese(II) and cobalt(II) with chloride ions in N,N-dimethylacetamide

Abstract

The formation of chloro complexes of manganese(II) and cobalt(II) has been studied in N,N-dimethylacetamide (DMA) by calorimetry and spectrophotometry. Formation constants, enthalpies and entropies of four mononuclear complexes [M^IICl_n]^(2–n)+(n= 1–4) have been determined for M = Mn and Co at 298 K and for M = Mn at 318 K. In spite of the similar solvent properties of DMA and N,N-dimethylformamide (DMF), overall formation of [MCl₄]^2– is strongly favoured and less endothermic in DMA than in DMF. Electronic spectra of the Co^II complexes obtained at 278, 298 and 318 K indicate an octahedral geometry for [Co(DMA)₆]²⁺ and tetrahedral geometries for [CoCl(DMA)₃]⁺, [CoCl₂(DMA)₂], [CoCl₃(DMA)]^– and [CoCl₄]^2– at each temperature, though an additional solvation equilibrium is observed for the monochloro complex: [CoCl(DMA)₅]⁺⇌[CoCl(DMA)₃]⁺+ 2DMA. The relatively large values of ΔH^°₁ and ΔS^°₁ suggest that extensive desolvation occurs on formation of the monochloro complex. Thus, the coordination geometry primarily alters at an earlier step (n= 1) in DMA than in DMA (n= 2). For the Mn^II system, large ΔH^°_n and ΔS^°_n values (n= 1, 2) are also diagnostic of an earlier transition from octahedral to tetrahedral geometry in DMA. The results show that the stability and structure of the complexes are strongly controlled by sterically hindered solvation of DMA molecules towards the metal ion, leading to the more favourable, less endothermic complexation and the tendency to form a less-crowded solvation structure. The steric effect of DMA is more pronounced in the order Mn^II < Co^II < Ni^II.