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The kinetics of the solvolytic aquation of trans-[Ru (3-Me Py)4Cl2] was studied spectrophotometrically in water – isopropanol in the range (30–90% v/v), and water acetonitrile in the range (10–70% v/v), and in the temperature range 50–65 °C. Plots of log k versus the reciprocal of the relative permittivity and Grunwald–Winstien gave non-linear plots. This non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. The plot of log k versus water concentration was also non linear; evidence for the presence of a S N 1 mechanism. However, extrema in the variation of enthalpy ΔH* and entropy ΔS* of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of ΔH* versus ΔS* were obtained and the iso- kinetic temperature indicates that the reaction is entropy controlled.

The solvolysis of the title complex is slower in 0 to 70% v/v aqueous dioxan than in aqueous acetonitrile at 40, 45, 50 and 55° C, a fact that is attributed to the higher basicity and ionizing power of the latter solvent mixtures. The observed non-linearity for logk (solvolysis) versus 1/D plots was interpreted in terms of the relative stability of ground and transition state species. The calculated Gibbs free energy for transfer of the species in the transition state is consistent with the extreme stretching of the chlorinecobalt bond, a result which confirms the interchange dissociative mechanismId for the complex under study.

The solvolysis of thetrans-[Co(4-Mepy)4Cl2]ClO4 complex was studied in 0 to 70% v/v H2O: MeOH mixtures at 40, 45, 50 and 55 °C. The high negative ΔS* values found for the complex cation under investigation, relative to that oftrans- [Co(py)4Cl2]+ reported in the literature, were attributed to the substituent methyl groups. The free energies of transfer of both the ground and the transition states were calculated from which the dominant effect of the solvent on the transition state is apparent.

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]2[ReCl6]. 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 −ΔGto(1) transition >−ΔGto(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)

The solvolysis oftrans-[Co(4-Etpy)4Cl2]ClO4, was followed spectrophotometrically in water/isopropanol at different temperatures. The activation energy varied nonlinearly with the mole fraction of the co-solvent,χ2. The plot of logk versus Ds−1 was also non-linear. These features were attributed to the differential solvation of the initial and transition states. On plotting ΔH† versus ΔS†, the points fall very close to straight line. The isokinetic temperature was found to be 334K, indicating that the solvolysis reaction is controlled by ΔS† and not ΔH†. The change in ΔH† and ΔS† with the mole fraction of the cosolvent shows extrema at the composition range where changes in solvent structure occur. The influence of the solvent structure on the complex ion in the transition state dominates over that in the initial state, where −ΔGt0[Co(4-Etpy)4Cl]2+>−ΔGt0[Co(4-Etpy)4Cl2]+.