Kinetic and Mechanistic Studies of Uncatalysed Substitution of Coordinated Cyanide in Hexacyanoferrate(II) by Phenylhydrazine

The kinetics of uncatalyzed reaction between hexacyanoferrate(II) and 2,2'-Bipyridine (further designated as Bipy) has been followed spectrophotometrically by monitoring the increase in absorbance at 400 nm, the λmax of cherry red complex, [Fe(CN)4Bipy] 2– as a function of pH, Ionic strength, temperature and concentration of reactants. In this reaction, the coordinated cyanide ion in hexacyanoferrate(II) get replaced by incoming ligand Bipy under the following specified reaction conditions. Temperature = 30+0.1 o C, pH = 3.8+0.02 and I = 0.10 M (KNO3). The uncatalyzed reaction between [Fe(CN)6] 4– and Bipy is very slow. The reaction follows first order dependence each in [Fe(CN)6] 4– and [Bipy]. It was found that the initial rate varies linearly with increasing concentration of hexacyanoferrate(II), keeping other variables fixed at an optimum value. The initial rate is also found to increase in the beginning with increasing concentration of [Bipy] up to [Bipy] < 1.5×10 -4 M, passes through a maximum and then finally falls. The effect of pH, ionic strength on initial rate have also been studied and explained. The activation parameters for reaction have been evaluated. A most plausible mechanistic scheme has been proposed based on the experimental observations.