Kinetics of the solvolysis of the trans-dichlorobis(1,2-diaminoethane)cobalt(III) ion in water + t-butyl alcohol mixtures

Abstract

The kinetics of the solvolysis of the 1,6-[Coen2Cl2]+ ion in mixtures of water with t-butyl alcohol have been investigated for concentrations of the alcohol up to a mole fraction of 0.16 for a range of temperatures. Values for the enthalpy and entropy of activation show an extremum at the same mole fraction where the relative partial molar volume of t-butyl alcohol shows a minimum; both also show an inflection at the concentration of t-butyl alcohol where the mixtures have a maximum in their ultrasonic absorption. In general, plots of log(rate constant) against the reciprocal of dielectric constant are curves for the solvolysis of 1,6-[Coen2Cl2]+ in water + cosolvent mixtures, and the application of a free-energy cycle to this solvolysis in water + t-butyl alcohol mixtures shows that changes in solvent structure have a greater effect on the pentacoordinated cobalt ion in the transition state than on the hexacoordinated ion in the initial state. These results are compared with those for the solvolysis of other complex cations in water + cosolvent mixtures, particularly where the cosolvent is t-butyl alcohol.