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 (k^s_obs) of this reaction decreases with increasing mole fraction (X_org) of the organic cosolvent, the effect being less marked with increasing hydrophobicity of the alcohols. The plots of log k^s_obsvs.D^–1_s(D_s= bulk dielectric constant) are curved. The plots of log k^s_obsvs. 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 (D_s= 50) at 50 °C the plot of log k^s_obsvs.X_org 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 (X_EG= 0.36) shows positive deviation indicating its better solvating ability. The observed variation of the rate constant on solvent composition (log k^s_obs= log k^w_obs– a X_org+bX²_org) for a given aquo-organic cosolvent is interpreted in terms of a preferential solvation model. The plots of activation enthalpy and entropy vsX_org 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 (k_H2O/k_D2O= 12.6) at 50 °C, which is slightly lower than the value for several other analogous alkylamine complexes.