Thermochromic behaviour and cobalt(II) bromide complex equilibrium in low temperature melting acetamide–ammonium nitrate–water mixtures

Abstract

The absorption spectra of cobalt(II) bromide in molten NH4NO3·2.61H2O, NH4NO3·CH3CONH2·1.61H2O and NH4NO3·2.61CH3CONH2 containing variable amounts of ammonium bromide were investigated at different temperatures from 45 to 75 °C. In the absence of bromide the solutions show spectra typical for octahedral coordination of cobalt(II). Addition of bromide caused a shift of the absorption maximum toward lower energies and an increase of the overall molar absorption coefficient with the increasing bromide concentration. The position of the absorption maximum and the intensity of absorption indicate tetrahedral or severely distorted octahedral co-ordination. Themolar absorption coefficient of the cobalt(II) species changeswith temperature as a result of changes in coordination geometry. By an analysis of the spectra with a suitable computer program itwas found that the following complexes were formed: [Co(NO3)4]2−, [Co(NO3)2Br2]2− and [CoBr4]2−. Stability constants of these complexes in NH4NO3·2.61CH3CONH2 have been calculated at 45 °C.