Kinetics and mechanism of base hydrolysis of cis-(ammine) bis (ethylenediamine) (substituted salicylato) cobalt(III) complexes

The kinetics of base hydrolysis of cis-(ammine) bis (ethylenediamine) (substituted salicylato) cobalt(III) complexes, [Co{CO₂C₆H₃(X)(OH)}(NH₃)(en)₂]²⁺ (X = H, 5-SO₃⁻, 5-Br, 5-NO₂ and 3-NO₂), have been investigated in aqueous medium of I = 1.0 M. The phenate species, [Co{CO₂C₆H₃(X)(0)}(NH₃)(en)₂]⁺, have been found to undergo base hydrolysis via two path ways, i.e. one zero order in [OH⁻] (k₁ path) and another first order in [OH⁻] (k₂ path). The alkali independent rate constant (k₁) increases with the basicity of the phenate group and the plot of log k₁ against pK_OH of the complexes is a straight line with a positive gradient of 0.98±0.03. In contrast to this the plot of log k₂ against pK_OH has a gradient of −0.15±0.02. The latter correlation indicates that the electron withdrawing substituents in the salicylate moiety enhance the rate of base hydrolysis in the alkali dependent path. The reaction is not subject to imidazole catalysis in both k₁ and k₂ paths. Substantially high positive values of ΔS^≠ for both the paths are observed. Several mechanistic possibilities have been considered. SN₁CB mechanism involving the rate limiting CoO bond fission in the triagonal bipyramidal transition state appears to be best suited for both the paths. For the alkali independent path the unbound phenate group is suggested to generate the reactive conjugate base by abstracting the NH proton from the coordinated amine.