Initial state and transition state solvation effects in the oxidation of iodide and catechol by hexachloroiridate(IV) in binary aqueous solvent mixtures

Rate constants are reported for hexachloroiridate(IV) oxidation of iodide in methanol–, ethanol–, t-butyl alcohol–, ethylene glycol–, glycerol–, acetone–, acetonitrile–, and dimethyl sulphoxide – water solvent mixtures, and for the hexachloroiridate(IV) oxidation of catechol in methanol–water mixtures. With the aid of ancillary solubility data and measurements, solvent effects on reactivity have been analysed into initial state and transition state components. In the latter, there are probably both electron transfer and diffusion contributions since it is not possible to identify uniquely the separate kinetic steps associated with the activation process. In these redox systems, however, transition state solvation changes dominate. This conclusion is compared with initial state – transition state effects in the reaction of catechol with hexahydroxoantimonic acid, in the bromate oxidation of bromide, the peroxodisulphate oxidation of iodide, and with patterns established earlier for substitution at inorganic centres. In view of some disagreements and uncertainties in the literature concerning the number of molecules of water of crystallisation in potassium hexachloroiridate(III) and hexachloroiridate(IV), this matter, of considerable importance to hexachloroiridate transfer parameter estimation, is briefly reviewed.