The kinetics of some isotopic exchange reactions of anionic ruthenium-(II), -(III), and -(IV) chloro-complexes in concentrated aqueous H36Cl solutions

Abstract

The kinetics of the apparent isotopic exchange reaction RuCl63–+36Cl–⇆ RuCl536Cl3–+ Cl– have been studied and rate parameters obtained. Increase of the total hydrochloric acid concentration from 6·0 to 11·75 M causes a decrease in the rate of exchange which, in this acid concentration range, is considerably less then the known rate of aquation of RuCl63– in more dilute acid. The activation enthalpy of the exchange in both 6·0 and 11·3M-hydrochloric acid is ca. 25 kcal. mole–1, which indicates that the overall exchange of chloride ions is controlled by the aquation of RuCl63–. The low exchange rates are correlated with the considerable reduction in water activity at high acid concentrations, and this is taken as evidence for the operation of a genuine SN2 type mechanism in the acid hydrolysis of RuCl63–. The effect of producing small amounts of lower oxidation states of ruthenium in RuCl63––36Cl– exchange solutions was investigated. The results indicated that, whereas ruthenium(II) does not cause noticeable catalysis, ruthenium(I) may be extremely active. Irradiations with 2537 and 3660 A light showed that the RuCl63––36Cl– exchange system is not strongly photosensitized by these wavelengths. The small acceleration observed with 3660 A light may arise from photoaquation of RuCl63–.Isotopic chloride-ion exchange experiments were carried out on 0·025M-ruthenium(II) and mononuclear ruthenium(IV) solutions in ca. 11M-hydrochloric acid at 25°. Infinite-time activity distributions indicated that the principal chlororuthenium species undergoing exchange in these solutions were RuCl42–, for ruthenium(II), and a RuCl62––RuOHCl52– equilibrium mixture, for ruthenium(IV). The rates of exchange revealed that the order of ligand lability in the higher (anionic) chloro-complexes of ruthenium is Ru(II) > Ru(III) > Ru(IV). The most important factors which contribute to (i) the variation in rates of solvolytic aquation of Group VIII transition-metal hexachloro-complexes, (ii) the occurrence of depressed rates of aquation at high acid concentrations, and (iii) direct chloride–chloride substitution paths in metal chloro-complexes, are also discussed.