Solvolysis of cobaltammines

Abstract

The reactions studied were the aquaton of chloropentammine cobalt III chloride and the anation of aquopentanmine cobalt III ions with chloride ions in various e thanol—water rixtures.
First order kinetics were observed for the aquation reaction being first order in cobalt concentration. The rate of nquation decreased with increasing ethanol concentration in the solvent 9 i. a. with thcreasing dielectric constant. The activation energies and frequency £actos decre sed and then increased with increasing ethanol in the solvent. The entropy of activation for rquation in pure water as the solvent was found to be - 5.3 e.u. The antpy of activation decro:-ased and then increased with ethanol concentration 7 becoming positive at about thirty—five per cent by volurje of ethanol in the solvent.
There v;as no linear dependence between the rate of aquation and the decreasing dielectric constant 9 the solvent ionizing parameter Y and the concentration of water in the solvent. This non—linearity was ecplaincd in terms of ion pairing increasing the rate of aquation.
It was found that nitrate and sulphate ions increased the rate of aquation. The ion pair reacted faster than the free ions. It was found that the amount of ion pairing increased'n'ith increasing ethanol concentration. The rate of aquation was retarded by the presence of perchlorate ions. This retardation effect was eKplained in terms of changes in ionic strength.
The aquation rate data was explained in terms of an and an SN2 uechanisrn but there seemed to be more evidence in terms of an 5N2 riechanisu. The anation reaction was found to be second order, first order in cobalt concentration and first order in chloride ion concentration. The anation reaction was found to be dependent on ionic strength. The rate of anation increased with increas:ng ethanol concentratLon in the solvent and therefore decreasing dielectric constant. The activation energies and frequency factors first of all decreased and then increased for increasing ethanol concentration. The entitpy of activation for the anation reaction in water in the solvent was +2.3 e.u. . The entropy of activation first of all decreased and then increased with increasinc ethanol concentration.
At constant ion c strength, the ratio of observed rate constant for anetion to the chloride ion concentration decreased with increasing chloride ion concentration. This was taken as evidence for a dissociation mechanism. Non linear relationships
were obtained for plots of the logarithm of the rate constant fo: anation against the reciprocal of the dielectric constant and the 'einstein solvent ionizinr poier parameter Y. It was found that there was considerable ion association between the aquopentanuina cobalt 111 ions and chloride ions. The anation results were explained in terns of an S Nl 1P nechanisri,
The equilibrium positions vicre affected by temperature, ionic strength and ethanol concentration.
Less and less chloropentsuzrine cobalt III chloride were aquated with increasing temperature, increasing ethanol concentration and decreasing ionic strength.