Kinetics of substitution reactions of hexacoordinate cobalt(III) complexes with phenoxyacetic acid herbicides

Abstract

Phenoxy herbicides are proved to be potential chelating agents in the metal complexation showing substitution reactions following second order kinetics. The reactions of cis-[Co(en) 2 (H 2 O) 2 ] 3+ with a series of substituted phenoxyacetic acids proved to be biomolecular and second order rate constants (log k) values are found to be linearly dependent on the Hammett substituent constants (σ) of respective phenoxyacetic acids. The high value of regression coefficient (r = 0.82) indicates that reactivities of these herbicides depend on number, nature and position of substituent group in the benzene ring. Further, the kinetic and thermodynamic stability of these complexes are also interpreted on the basis of equilibrium and rate constant data. The rate and equilibrium constants, spectral changes and their linear correlations with Hammett substituent constants (σ) for both biologically active and inactive phenoxyacetic acids are of comparable magnitudes, thereby ruling out the possibility of chelation as a possible mode of action of these herbicides.