Kinetics and Mechanism of Formation of [Fe(Cn) 5 Inh] 3- by Ligand Substitution in Acidic Media (Inh = Isoniazid)

The kinetics and mechanism of formation of [Fe(CN)₅INH]^3- by a ligand substitution reaction between aquapentacyanoferrate(II) ion and isoniazid, (pyridine-4-carboxylic acid hydrazide) (INH) have been investigated spectrophotometrically in acidic media at 435 nm (Λ_max of product), under pseudo-first order conditions, using excess of INH over [Fe(CN)₅H₂O]^3-. The reaction was studied as a function of [Fe(CN)₅H₂O^3-], [INH], pH, ionic strength and temperature, by varying only one of these variables while other variables were held constant. The reaction exhibited first-order kinetics in [INH] while it was independent of [Fe(CN)₅H₂O^3-] in the chosen concentration range. The effect of temperature on the reaction rate yielded the activation parameters. The most plausible mechanism consistent with the experimental results has been proposed.