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The formation of an antitubercular complex [Fe(CN)5(INH)]3− through mercury(II)-catalyzed ligand substitution reaction: A kinetic and mechanistic study



The kinetics and mechanism of the formation of an antitubercular complex [Fe(CN)5(INH)]3− based on the substitution reaction between K4[Fe(CN)6] and isoniazid (INH), i.e., isonicotinohydrazide, catalyzed by Hg2+ in aqueous medium was studied spectrophotometrically at 435 nm (the λmax of the golden-yellow-colored complex [Fe(CN)5(INH)]3−) as a function of pH, ionic strength, temperature, and the concentration of the reactants and the catalyst. The replacement of coordinated CN in [Fe(CN)6]4− was facilitated by incoming ligand INH under the optimized reaction conditions: pH 3.5 ± 0.02, temperature = 30.0 ± 0.1°C, and ionic strength I = 0.05 M (KNO3). The stoichiometry of the reaction and the stability constant of the complex ([Fe(CN)5(INH)]3−) have been established as 1:1 and 2.10 × 103 M, respectively. The rate of catalyzed reaction was found to be slow at low pH values, to increase with increasing pH, to attain a maximum value at 3.50 ± 0.02, and finally to decrease after pH > 3.5 due to less availability of H+ ions needed to regenerate the catalytic species. The initial rates were evaluated for each variation from the absorbance versus time curves. The reaction was found to be pseudo-first order with respect to [INH] and first order with respect to [Fe(CN)64−] at lower concentration, whereas it was found to be fractional order at higher [INH] and [Fe(CN)64−]. The ionic strength dependence study showed a negative salt effect on the rate of the reaction. Based on experimental results, a mechanism for the studied reaction is proposed. The rate equation derived from this mechanism explains all the experimental observations. The evaluated values of activation parameters for the catalyzed reaction suggest an interchange dissociative (Id) mechanism. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 398–406, 2012