Kinetics and mechanism of mercury catalysed ligand exchange reaction between hexacyanoferrate(II) and phenanthroline in aqueous medium

Kinetic and mechanistic studies of Hg(II)-catalysed exchange of co-ordinated cyanide in hexacyanoferrate(II) by phenanthroline (Phen) were performed by following the appearance of the coloured complex [Fe(CN)4 Phen]3– at 528 nm, temperature = 25 ± 0.10 0C, pH = 3.0 ± 0.02. The effects of pH, [Fe(CN)6]4– and [Phen] concentrations on the reaction rate have been studied and explained. The rate shows variable order dependence on [Fe(CN)6]4–. The effects of dielectric constant, water content, temperature and ionic strength of the medium on the reaction rates have been used to visualise the formation of a polar activated complex and suggest an interchange dissociative mechanism for the catalysed reaction. The varying catalytic activity of Hg(II) as a function of concentration has also been studied and explained in detail. Activation parameters of both catalysed and uncatalysed reactions has were derived and supported the proposed mechanistic scheme.

Kinetics and Mechanism of Uncatalysed Substitution of Cyanide in Hexacyanoferrate(Ii) by Isoniazid

The kinetics and mechanism of the uncatalysed ligand substitution reaction between potassium ferrocyanide, K4[Fe(CN)6] and isoniazid (INH) in aqueous medium has been investigated spectrophotometrically at 435 nm, i.e. the Λmax of the golden yellow complex, [Fe(CN)5(INH)]3-, as a function of [Fe(CN)64-] and [INH] under the conditions: pH = 3.5 ± 0.02, temperature = 30 ± 0.1°C and ionic strength, I = 0.05 mol/l (KNO3). The reaction is first order each in [Fe(CN)64-] and isoniazid (INH) at low [INH]. The initial rate of reaction is determined from the slopes of absorbance versus time plots. As [INH] increases, the rate of reaction increases, passes through a maximum and then falls, suggesting that the course of the reaction is different at low and high [INH]. A tentative mechanism has been proposed, which is further supported by the results of studies, such as variation in [Fe(CN)64-], effect of temperature and pH of the reaction medium. The composition of the complex formed in the present reaction has been established to be 1:1 by the mole ratio and slope ratio methods. A repetitive spectral scan is also provided in support of the exchange of coordinated cyanide ions by isoniazid in hexacyanoferrate(II).