Influence of Solvent upon Rates of Ion Pair Reactions: Studies on the Aquation of Bromopentamminecobalt(Iii) Perchlorate in Binary/Water-Ethan-1,2-Diol Mixtures

The kinetics of the aquation of bromopentamminecobalt(III) ion have been investigated in malonate media containing different percentages of ethan-1,2-diol (up to 50%). The ion pair rate constant k_ip, has been determined over a wide range of malonate buffer compositions using the Wyat and Davies treatment. The thermodynamic and extrathemodynamic correlations existing between those of the association reaction (ΔG°_ass, ΔH°_ass, ΔS°_ass) and of the ion pair aquation reaction (ΔG^≠_ip ΔH^≠_ip, ΔS^≠_ip) have been investigated. The kinetic solvent effects are examined in terms of the dielectric constant and water concentration effects on the ion pair rate constant. Also, the solvent effects are analysed in terms of Kamlet–Taft solvatochromic parameters and are applied successfully in mixed aqueous – ethan 1,2-diol mixtures. However, the extrema found in the change of ΔH^≠_ip and ΔS^≠_ip with the mole fraction of the co-solvent have been correlated well with that found in the change of the physical properties of the solvent–water mixtures with the solvent structure. Small changes of ΔG^≠_ip with the mole fraction of the solvent are attributed to a quasi-mirror image compensation between ΔH^≠_ip and ΔS^≠_ip.