Mechanistic studies on the reactions of pentaaquahydroxochromium(III) ion with 2,2?-bipyridine and 1, 10-phenanthroline

Abstract

Kinetic studies in water-ethanol solution on the replacement of the aqua-ligands in [Cr(H2O)5(OH)]2+ by 2,2′-bipyridine (bipy) and 1, 10-phenanthroline (phen) have been made spectrophotometrically. In both the cases the reaction proceeds by a path which is first order with respect to the base. The reaction is catalysed by NO3− and the rate increases with the increase of pH. We propose the rate law $$k_{obs} = k_1 k_2 \left[ L \right]/\left( {k_{---1} + k_2 \left[ L \right]} \right)$$ where k1 is the water dissociation rate of the substrate complex, k−1 is the aquation and k2 is the ligand capturing rate of the pentacoordinate intermediate [Cr(H2O)4(OH)]2+. Experimental results are consistent with a dissociative mechanism. Variation of dielectric constant of the medium was used to verify the mechanistic conclusion in the reaction with bipy.