The study of kinetics and mechanism of ligand substitution reactions of [PdPar(OH)] − and [Cd(Par) 2] 2− (Par=4-(2-pyridylazo)resorcinol) with cyanide ions by stopped flow technique

Abstract

The reaction of [PdPar(OH)] − with cyanide ion has been followed at 510 nm (λ max of [PdPar(OH)] −) while the reaction of [Cd(Par) 2] 2− with cyanide ions has been followed at 495 nm (λ max of [Cd(Par)] or [Cd(Par) 2]) by the stopped flow technique at pH= 11.0 ± 0.02, I=0.1 M (NaClO 4) and temperature= 25 ± 0.1 °C under pseudo-first-order conditions taking cyanide in large excess. The reaction between [PdPar(OH)] − and cyanide ion follows first-order kinetics in both [CN −] and [PdPar(OH)] −. The reactions of [Cd(Par)] and [Cd(Par) 2] with cyanide ions also follow first-order kinetics in each reactant at higher cyanide concentrations. The reactions of [PdPar(OH)] and [Cd(Par) 2] with cyanide ions exhibit zero-order dependences in [CN −] at extremely low cyanide concentrations. The reverse reaction between [Pd(CN) 4] 2− and Par, and [Cd(CN) 4] 2− and Par follow first-order kinetics in each reactant and an inverse first-order dependence in cyanide ion. It is inferred that the fourth step is the rate determining one in a proposed five step mechanistic scheme. Additional studies viz. effect of ionic strength, pH dependence and temperature dependence were also conducted and support the proposed mechanistic scheme.