Cobalt(II)-halide association equilibria in ammonium nitrate-dimethyl sulfoxide melts

The cobalt(II)-chloride association equilibria in ammonium nitrate-dimethyl sulfoxide (DMSO) melts of the type NH4NO3·xDMSO at five different compositions in the range 1.0 ≤ x ≤ 5.0 have been investigated by the spectrophotometric method. Absorption spectra of cobalt(II) chloride solutions containing variable amounts of ammonium chloride at different temperatures from 35 to 65 °C have been recorded in the wavelength range of 400-800 nm. In the absence of chloride, the solutions show spectra typical for octahedral coordination of cobalt(II). Addition of chloride changed the absorption spectra to a tetrahedral or severely distorted octahedral coordination. Stability constants for the complexes [Co(NO3)2(DMSO)2], [Co(NO3)2X2]2− and [CoCl4]2−, and the species spectra for all complexes are reported. The effect of varying the DMSO/NH4NO3 mole ratio is discussed. A sharp increase of the stability constants around x = 2 is correlated to the competition between the solvent molecules or ions and the cobalt(II) ions for the ligand and to the solvation of NH4+ with two DMSO.