Cobalt(II) Chloride Complex Formation in Acetamide–Calcium Nitrate Tetrahydrate Melts

The absorption spectra of Co(II) chloride complexes, containing variable concentrations of chloride ligand, in a molten mixture of 80 mol% acetamide-20 mol% calcium nitrate tetrahydrate, were studied at 313, 333, 353, and 363 K, in the wavelength range 400-800 nm. The melt contains three possible ligands (CH3CONH2, H2O, and NO3-) for competition with added chloride ligand. Addition of chloride caused a shift of the absorption maximum of octahedral cobalt( II) nitrate towards lower energies and pronounced changes in the shape of the initial spectrum of cobalt( II) nitrate. The effect of temperature changes on the molar absorption coefficient of the Co( II) species was dependent on the chloride concentration and was attributed to the structural changes occurring in the cobalt( II) species. The STAR and STAR FA programs were applied to identify the complex ionic species and to calculate the stability constants of Co( II) complexes formed in this solvent. The results indicate the highest probability of formation of the following complex species: Co(NO3)(4)(2-), Co(NO3)(2)Cl-2(2-), and CoCl42-. Stability constants of each complex were presented for the equilibria occurring at 313, 333, 353, and 363 K. Distribution of the Co( II) species was also calculated over the ranges of chloride concentration and temperature investigated.