The complex formation between cobalt(II) and chloride and bromide ions in molten calcium nitrate tetrahydrate at different temperatures has been studied by a spectrophotometric technique. Addition of halide ions to cobalt(II) nitrate solution in calcium nitrate tetrahydrate caused a pronounced shift of the absorption maximum toward lower energies and a large increase of absorption intensity, indicating a change from octahedral to a tetrahedral co-ordination. The change of co-ordination depends on temperature and halide concentration. Stability constants for the [Co(NO 3 ) 4 ] 2− , [Co(NO 3 ) 2 X 2 ] 2− and [CoX 4 ] 2− complexes at 40 and 70 • C and the corresponding species spectra are reported.
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