Influence of electrolyte dissociation upon rates of reactions. Part VII. Effects of sulphate and some dicarboxylate media upon rates of aquation of halogenopenta-amminecobalt(II) complexes and the rate of base hydrolysis of the acetatopenta-amminecobalt(II) complex

Abstract

Accelarations to the rates of aquation of chloropenta-amminecobalt(III)(at 35 °C) and bromopenta-amminecobalt-(III)(at 25 °C) complexes caused by some ion-pairing dicarboxylates and sulphate have been studied by spectrophotometry. Similar work with iodopenta-amminecobalt(III) has been carried out at 25 °C by means of ¹²⁵I release. The results are analysed in terms of ion-pair rate constants of aquation. The ratios of these rate constants to those of the free cobalt(III) ions are in the range 2—3·1 : 1, being mainly independent of the halide component and only slightly related to the ion-pair dissociation constants. The rate of base hydrolysis of the acetatopenta-ammine-cobalt(III) complex at 25 °C, obtained by following the rate of release of [1-¹⁴C]acetate, is retarded in dicarboxylate and sulphate media. There is a distinct proportionality between corresponding ion-pair rate and dissociation constants. The latter have been calculated from e.m.f. measurements on a glass electrode–calomel electrode cell (for dicarboxylate media) and from solubility measurements (for sulphate media).