Solvent effects on the initial and transition states upon solvolysis oftrans-dichlorotetra(4-ethylpyridine)cobalt(III) perchlorate in water+methanol mixtures

Summary The solvolysis of trans-Co(4-Etpy)₄Cl₂]⁺ (1) (Etpy=ethylpyridine) was investigated in water and in a wide range of water+methanol mixtures. The effect of temperature on the solvolysis was considered and the thermodynamic parameters of activation were calculated. The free energies of transfer of (1) in the initial state were calculated from the solubility of the complex Co(4-Etpy)₄Cl₂]₂ReCl₆]. The free energy of transfer of the cation (1) in the transition state was calculated using a free energy cycle. The effect of the solvent structure on (1) is dominant in the transition state, where –G _t ^o (1) transition >–G _t ^o (1) initial. Comparison of the present thermodynamic data with those obtained previously fortrans-Co(4-Mepy)₄Cl₂]⁺, (2), andtrans-Co(py₄Cl₂]⁺, (3) in the same solvent mixture, shows that the stability increases in the order: (1), (2)<(3) at low mole fraction of the co-solvent, contrary to expectation, while at high mole fraction the order of increasing stability is: (1)<(3)<(2). The results were interpreted on non-electrostatic grounds.