The Palladium(Ii)-Catalysed Ligand Exchange between Hexacyanoferrate(Ii) and 4-Cyanopyridine in Aqueous Acidic Medium

Abstract

The kinetics of the palladium(II)-catalysed ligand exchange of the hexacyanoferrate(II) complex ion with 4-Cyanopyridine (4-CNpy) in aqueous acidic medium have been investigated at 25.0 ∓ 0.1 °C, pH 3.2 ∓ 0.02 and ionic strength, I=0.1 M (KNO3). The reaction was followed spectrophotometrically by measuring the increase in absorbance of the intense cherry red complex [Fe(CN)54-CNpy] 3-, corresponding to metal-to-ligand charge transfer (MLCT) transitions, atλ.max 477 nm. The effects of pH, ionic strength, the concentrations of [Fe(CN)54-], Pd2+ and 4-CNpy on the reaction rate have been studied and analysed. The kinetic data suggest that substitution follows an interchange dissociative (Id) mechanism and occurs via formation of a solvent-bound intermediate. The effects of the dielectric constant of the medium on the reaction rate have been used to visualize the formation of a polar activated complex and an interchange dissociative mechanism for the reaction. A most probable mechanism has been proposed in order to interpret the kinetic data. Kinetic evidence is reported for the displacement of CN- by 4-CNpy in [Fe(CN)64-]. Activation parameters for the proposed reaction have been evaluated using an Eyring plot, and provided further support to the proposed mechanistic scheme.