Solvent-interchange reactions of the penta-amminecobalt(III) ions [Co(NH3)5(OH2)]3+ and [Co(NH3)5(dmf)]3+ in dimethylformamide (dmf)–water media

Abstract

The rate law for aquation of [Co(NH₃)₅(dmf)][ClO₄]₃ is first order with respect to the concentration of the cobalt(III) complex and independent of aluminium(III) or perchloric acid concentration, contrary to a previous report. Tracer experiments have shown more than 95% cobalt(III)–oxygen bond fission, and chromatographic analyses for the formato-complex indicated less than 1% carbon–nitrogen bond fission. It is concluded that aquation proceeds by water molecules replacing intact dimethylformamide (dmf) molecules. Solvent interchange between the aqua- and dimethylformamide complexes has been studied in dmf–water mixtures at 25, 45, and 65 °C. The activation enthalpy for aquation of the dmf complex is independent of the composition of the solvation shell of the complex. The dependence of rate and equilibrium constants on dmf concentration is interpreted in terms of a free dmf-co-ordinated dmf interaction at small dmf contents which decreased the aquation rate constants at a more rapid rate at small dmf contents than at larger dmf contents at all temperatures. This same interaction increased the equilibrium concentration of the dmf complex more rapidly at small dmf than at larger dmf contents at 65 and probably at 45 °C also. Other explanations for preferential solvation of the complexes are considered and rejected. The rate law for aquation is R_aq=K_aq′[Co(NH₃)₅(dmf)³⁺]x_H2O. The activation parameters for k_aq′ are satisfactorily constant over the range 0–0.7 mol fraction dmf, viz.ΔH^‡= 105 ± 2 kJ mol^–1 and ΔS^‡=–3 ± 7 J K^–1 mol^–1. The rate law for ligation is expressed as R_lig=k_lig′[Co(NH₃)₅(OH₂)³⁺]x_dmf. The activation parameters for k_lig′ are not constant; ΔH^‡_lig and ΔS^‡_lig both decrease with increasing dmf content (x_dmf).