. 61, 1361 (1983). Rate constants arc rcportcd for hexachloroiridate(IV) oxidation of iodidc in methanol-, cthanol-, t-butyl alcohol -, cthylene glycol-, glycerol-, acetone-, acetonitrile-, and dimcthyl sulphoxidc -water solvent mixtures, and for thc hcxachloroiridate(1V) oxidation of catcchol in methanol-watcr mixtures. With thc aid of ancillary solubility data and measurements, solvent effects on reactivity have been analysed into initial statc and transition statc components. In the lattcr, there arc probably both electron transfer and diffusion contributions since it is not possiblc 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 earlicr 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(II1) and hexachloroiridate(1V). this matter, of considcrable importance to hexachloroiridate transfer parameter estimation, is briefly reviewed.

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LE1 7RHRate constants are reported for the reaction between bromide ions and the complex ion [PdCl(tedien)]+ (tedien = NNN"N"-tetraethyldiethylenetriamine) in aqueous solution containing various concentrations of bromide salts. The first-order rate constant for the unimolecular substitution reaction decreases with increase in salt concentration, the effect being more marked through the series NBun4+ > NPr,+ > NEt,+ > NMe,+ > K+ N Na+. Comparison of rate data and solubilities of the complex chloride salt in 1 .O mol dm-3 KCI, [NEt,]CI, and [NBu4]CI indicates that the dominant salt effect operates on the transition state, where hydration and ion-size effects lead to a marked destabilisation.