The association of hydroxide ion with tris(ethylenediamine)cobalt(III) and tris(ethylenediamine)chromium(III) cations in dioxan–water and dioxan–deuterium oxide mixtures

Abstract

The equilibrium constant K for the association of tris(ethylenediamine)-cobalt(III) and -chromium(III) cations with hydroxide ion has been measured, mainly at 25·0°, by following the change in ultraviolet extinction coefficients of the complexes as a function of hydroxide ion concentration. At constant ionic strength, the equilibrium constants calculated are independent of the concentration of the complex or of the wavelength of the light, and affected only very slightly by changes in temperature. The value for the chromium complex is smaller than for cobalt, a phenomenon which might be partly responsible for the differences in rates of base hydrolysis of halogeno-ammine cations of the two metals. In the presence of dioxan, the extent of association for both complexes is greater, the equilibrium constants varying exponentially with the dielectric constants of the solvent mixture. The effect of replacing water by deuterium oxide is negligible for the cobalt complex, but quite appreciable for the chromium(III) cation, where there was a two- to three-fold increase in the observed ion-pair association constants. Deuteriation of the complex does not produce any noticeable effect.