Solubility of salts of hexachlororhenate(IV) ions with complex cations in water and in water + alcohol mixtures: free energies of transfer of the complex ions

Abstract

It is shown that when the solubilities of two salts of the same valence type with a common anion are known in water and in mixtures of water with a co-solvent, and when the free energies of transfer ΔG_t^⊖ from water into the mixture are known for one of the cations, ΔG_t^⊖ for the other cation can be calculated. Solubilities of {a,f-[Co(py)₄Cl₂]}₂[ReCl₆] in the mixtures water + Bu^tOH and water + methanol and the solubilities of {a,f-[Co(4Me-py)₄Cl₂]}₂[ReCl₆] and of {a,b-[Co(SCN)Cl(en)₂]}₂[ReCl₆] in water Bu^tOH have been measured at 25.0 °C (py = pyridine, 4Me-py = 4-methylpyridine, en = ethylenediamine). These values have been combined with literature data for the solubility of Cs₂[ReCl₆] and for ΔG_t^⊖(Cs⁺) in these mixtures to produce values for ΔG_t^⊖ for the complex cations. ΔG_t^⊖ Values found for a,f-[Co(py)₄Cl₂]⁺ in water + methanol, together with kinetic data for the solvolysis of the cation and with values for ΔG_t^⊖(Cl^–) in the same mixtures, have been used to derive values for ΔG_t^⊖ for the cation in the solvolytic transition state, [Co(py)₄Cl]²⁺. This confirms that the effect of solvation of the cation in the transition state dominates over that of the cation in the initial state.