Effect of solvent on the reactions of coordination complexes. Part 8.—Kinetics of solvolysis of cis-(chloro)(ethanolamine)bis(ethylenediamine)cobalt(III) in methanol–water, propan-2-ol–water and t-butyl alcohol–water

Abstract

The solvolysis of cis-(chloro)(ethanolamine)bis(ethylenediamine) cobalt (III) in aqueous methanol, propan-2-ol and t-butyl alcohol results in the formation of the chelated ethanolamine complex, cis-[Co(en)2(NH2CH2CH2OH)]3+. The pseudo-first-order rate constant (ksobs) of this reaction decreases with increasing mole fraction (Xorg) of the organic cosolvent, the effect being less marked with increasing hydrophobicity of the alcohols. The plots of log ksobsvs.D–1s(Ds= bulk dielectric constant) are curved. The plots of log ksobsvs. Grunwald Winstein ‘Y’ parameter are nearly linear for CH3OH–H2O and propan-2-ol-H2O, but appreciably curved t-butyl alcohol–H2O. Under isodielectric conditions (Ds= 50) at 50 °C the plot of log ksobsvs.Xorg for CH3OH–H2O, C2H5OH–H2O, (CH3)2CH—OH–H2O, (CH3)3C—OH–H2O and (CH3)2CO–H2O in water-rich media is a straight line from which the data point for ethylene glycol–H2O (XEG= 0.36) shows positive deviation indicating its better solvating ability. The observed variation of the rate constant on solvent composition (log ksobs= log kwobs– a Xorg+bX2org) for a given aquo-organic cosolvent is interpreted in terms of a preferential solvation model. The plots of activation enthalpy and entropy vsXorg exhibit maxima and minima, thereby indicating that solvent structural changes play a significant role even for this intramolecular dissociative interchange process. The solvolysis of the complex in 99 % D2O yielded a solvent isotope effect (kH2O/kD2O= 12.6) at 50 °C, which is slightly lower than the value for several other analogous alkylamine complexes.