Kinetics and Mechanism of Substitution of Aqua-ligands from cis-Diaquo-bis(ethylenediamine)cobalt(IIl) Ion by DL-Alanine in Water-Ethanol Mixture

Department of Chemistry, University of Burdwan, Burdwan-713 104

Manuscript received 20 July 1984, revised 14 January 1985, accepted 31 July 1985

The kinetics of substitution of aquo ligands from $cis$-diaquo-bis(ethylenediamine) cobalt(III) ion by DL-alanine has been studied spectrophotometrically. The following rate law has been established

                    $\frac{d[Co(en{)}_2(alanine){]}^{2+}}{dt}$=$\frac{k_a{K}_E[Co(en{)}_2(H_{2}O{)}_2]total[ala.H]}{1+K_E[ala.H]}$

in the $p$H range 3.2 to 4.2, where, K_E is the ion-pair equilibrium constant and $k_a$ is the rate constant for conversion of outersphere complex to innersphere complex. The reac­tion rate is found to be $p$H dependent in the range 3.2 to 4.2. The anation rate and acti­vation parameters have been calculated and compared with isotopic water-exehange reac­tion. A mechanism involving outersphere association between the reacting species, followed by dissociative interchange of the outersphere complex to the product has been suggested.