Solvation and complexation equilibria of nickel(II) thiocyanato complexes in N,N-dimethylacetamide

Abstract

The complexation of nickel(II) with thiocyanate (SCN^–) ions has been studied by titration spectrophotometry and calorimetry in N,N-dimethylacetamide (DMA) containing 0.2 mol dm^–3(n-C₄H₉)₄NBF₄ as a constant ionic medium at 298 K. All the experimental data were well explained in terms of the formation of four mononuclear [Ni(NCS)_n]^(2–n)+(n= 1–4) complexes and their formation constants, enthalpies and entropies were determined, together with their intrinsic electronic spectra. The intrinsic spectra were also extracted at 278 and 318 K, and their temperature dependence clearly demonstrated the presence of solvation equilibria, i.e. an equilibrium between two structural isomers with different numbers of bound solvent molecules. This is not the case for the Ni^II–SCN^– system in N,N-dimethylformamide, where all the mono-, di-, tri- and tetra-thiocyanato complexes have a single structure of an octahedral six-coordination. These facts imply that the coordination of solvent molecules in the first solvation shell of the metal ion is strongly sterically hindered with DMA.