Thermodynamics and Structures of Nickel(II) Chloro Complexes in N, N-Dimethylacetamide

Formation of nickel(II) chloro complexes in Y.JV-dimethylacetamide (DMA) has been studied by spectrophotometry at 278, 298, and 318 K and by calorimetry at 298 and 318 K. Formation constants, reaction enthalpies and entropies, and electronic spectra for four mononuclear complexes [NiCl_n]^(2-n)+ (n = 1-4) have been determined at each temperature. The obtained spectra indicate an octahedral geometry for [Ni(DMA)₆]²⁺ and tetrahedral ones for [NiCl₃(DMA)]^- and [NiCl₄]^2-. The [NiCl]⁺ complex, however, shows a distinct absorption at 461 nm, and [NiCl₂] shows one at 489 nm, which are markedly different from those in similar donor solvents such as N-dimethylformamide (DMF). These two complexes are strongly thermochromic, suggesting that the following equilibria may hold for the mono- and dichloro complexes; [NiCl(DMA)₅]⁺ ⇄ [NiCl(DMA)₄]⁺ + DMA and [NiCl₂(DMA)₃] = [NiCl₂(DMA)₂] + DMA. Overall formation of [NiCl₄]^2- is strongly favored and much less endothermic in DMA than in DMF. The stepwise &H° and ΔS° values (n = 1, 2) are larger in DMA, being consistent with the strong desolvation in earlier steps of complexation. The molar heat capacity of complexation, ΔC°, estimated from temperature dependence of ΔH°n and ΔS°n, is positive for n = 1 and negative for n = 3, which is attributed to the shift of the solvation equilibria with temperature. These results indicate that steric requirements of the acetyl methyl groups of DMA molecules coordinated to the nickel(II) ion lead to (i) weaker solvation and thus more favored complexation and (ii) formation of the unusual coordination structures, five-coordinate [NiCl(DMA)₄]⁺and [NiCl₂-(DMA)₃] and tetrahedral [NiCl₂(DMA)₂].