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 RuCl₆^3–+³⁶Cl^–⇆ RuCl₅³⁶Cl^3–+ 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 RuCl₆^3– 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 RuCl₆^3–. 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 S_N2 type mechanism in the acid hydrolysis of RuCl₆^3–. The effect of producing small amounts of lower oxidation states of ruthenium in RuCl₆^3––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 Å light showed that the RuCl₆^3––³⁶Cl^– exchange system is not strongly photosensitized by these wavelengths. The small acceleration observed with 3660 Å light may arise from photoaquation of RuCl₆^3–.

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 RuCl₄^2–, for ruthenium(II), and a RuCl₆^2––RuOHCl₅^2– 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.