Effect of solvent on the reaction of coordination complexes. Part 19.—Base hydrolysis of (αβS)-(o-methoxy benzoato)(tetraethylenepentamine)-cobalt(III) in aquo-organic solvent media

Abstract

The kinetics of base hydrolysis of (αβS)-(o-methoxy benzoato)(tetren)cobalt(III) have been investigated in aquo-organic solvent media [0–70%(v/v) cosolvents] at 10 ⩽t/°C ⩽ 40 (l= 0. 01 mol dm^–3) using methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, tert-butyl alcohaol, ethylene glycol, 2-methoxyethanol, acetone, acetonitrile, 1, 4-dioxane and dimethyl sulfoxide as cosolvents. The second-order base hydrolysis rate constant increased non-linearly with increasing mole fraction (X_org) of all cosolvents, except for the ethylene glycol–water system; ethylene glycol had a rate-retarding effect. The transfer Gibbs energy of the transition state (TS) relative to that of the initial state (IS), for transfer of species from water to mixed solvent varied non-linearly with D^–1_s and X_org, reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and the initial state were governed by the preferential solvation effect. The solvent stabilisation of the initial state and the transition state has been assessed for the methanol–water and ethanol–water systems by combining the solubility data of the dithionate salt of the complex with the transfer Gibbs energy data for S₂O₆^2–. The thermodynamic parameters (ΔH^‡ and ΔS^‡) were sensitive to the structural changes in the bulk solvent phase.