Kinetics and mechanism of formation of the complex [Ru(CN)₅INH]³⁻ through the ligand substitution reaction between the aquapentacyanoruthenate(II) anion and isoniazid

The formation kinetics of the complex, [Ru(CN)₅INH]³⁻, formed through the ligand substitution reaction between isoniazid (INH) and aquapentacyanoruthenate(II) ([Ru(CN)₅H₂O]³⁻), have been investigated, under pseudo first-order conditions, as a function of concentrations of [INH] and [Ru(CN)₅H₂O]³⁻, ionic strength and temperature at pH = 4.0 ± 0.02 in 0.2 M NaClO₄ spectrophotometrically at 502 nm (λ_max of intense yellow colour product [Ru(CN)₅INH]³⁻) corresponding to metal-to-ligand charge-transfer transitions, in aqueous medium. The pseudo first-order condition was maintained by taking at least 10% excess of [INH] over [Ru(CN)₅H₂O]³⁻. The stoichiometry of the reaction product was found to be 1:1 which was further supported and characterized using elemental analysis, infrared, nuclear magnetic resonance and mass spectrometric techniques. Thermodynamic and kinetic parameters have also been computed, using the Eyring equation, and the values of ΔH^≠, E_a, ΔG^≠ and ΔS^≠ were found to be 47.3, 49.8, –8.62 and 187.6 J K⁻¹ mol⁻¹, respectively. The reaction was found to obey first-order kinetics with respect to [INH]. It exhibited a negative salt effect on the rate upon variation of ionic strength of the medium. A tentative mechanistic scheme was proposed on the basis of experimental findings.