The complex formation equilibria for the nickel(II)-bromide and -iodide systems in dimethyl sulfoxide and the bromide system in N,N′-dimethylpropyleneurea have been studied calorimetrically and spectrophotometrically at 298 K. An ionic strength of 0.1 mol dm−3 was kept constant using tetrabutylammonium perchlorate as supporting electrolyte. Two stable mononuclear bromo-complexes are formed in dilute N,N′-dimethylpropyleneurea solution with β1
=
(2.0 ± 0.4)
× 103 mol−1 dm3 and β2
=
(7.2 ± 1.5)
× 105 mol−2 dm6, whereas nickel(II) does not form any measurable complexes with bromide and iodide ions in dimethyl sulfoxide solution. The structures of the solids precipitating from concentrated solutions of nickel(II)-bromide and -chloride in the respective solvents have been determined by X-ray diffraction. The solid precipitating from a concentrated N,N′-dimethylpropyleneurea solution of nickel(II) bromide is a stable pseudotetrahedral bis(N,N′-dimethylpropyleneurea)dibromonickel(II) complex. Precipitation of nickel(II) chloride from a dimethyl sulfoxide solution results in hexakis(dimethyl sulfoxide)nickel(II) tetrachloronickelate(II), which is gradually transformed to hexakis(dimethyl sulfoxide)nickel(II) chloride, the most enthalpy stabilised complex. Hexakis(dimethyl sulfoxide)nickel(II) bromide precipitates from a concentrated dimethyl sulfoxide solution of nickel(II) bromide.
