Reaction mechanisms in molten salts. Part II. Ligand replacement and redox reactions of cobalt(III) complexes dissolved in hydrogen sulphate melt and in 100% sulphuric acid

Abstract

Changes in the charge transfer and d–d bands of a number of cobalt(III) ammine and ethylenediamine complexes dissolved in 100% sulphuric acid at 130–152°C and in molten NaHSO₄–KHSO₄ at 130–152°C indicate that finally there is complete ligand substitution and reduction to cobalt(II). The ligands water, chloride, nitrate, oxalate, carbonate, and nitro are replaced much faster than are ammonia or ethylenediamine. For a series of (solvated) ammines, it is found that the rate of decomposition to cobalt(II) in both solvents is in the order: Co(NH₃)₆³⁺ < Co(NH₃)₅³⁺ < Co(NH₃)₄³⁺ < Co(NH₃)₃³⁺ < Co(NH₃)₂³⁺. The results indicate that the principal reaction path in the decomposition of the hexa-ammine is ligand replacement by solvent ligands before reduction of the central cobalt(III) and that the rate of this reduction is enhanced as more ammine groups are replaced in the co-ordination sphere.