Ag(I)-Promoted Substitution of Cyanide from Hexacyanoferrate(II) with Pyrazine: A Kinetic and Mechanistic Study

The kinetic spectrophotometric study of Ag(I)-promoted production of [Fe(CN)5PZA]3– was performed by monitoring the absorbance at 440 nm, the λmax of yellow-colored complex [Fe(CN)5PZA]3–. The formation of [Fe(CN)5PZA]3– proceeds via the Ag(I) catalyzed substitution of cyanide from [Fe(CN)6]4– with chromogenic ligand pyrazine (PZA). Pseudo-first-order conditions were used to study the reaction rate as a function of ionic strength, [Ag+], [PZA], temperature, pH, and [Fe(CN)64−] by varying one parameter at a time. According to the effect of reaction parameters on the initial rate, it was found that [Ag+] and pH strongly affect the reaction rate. The reaction displays variable order kinetics with [Fe(CN) ₆^4 - ] and [PZA] in the studied concentration ranges. Inverse trend in the initial rate with ionic strength exhibits a negative salt effect. An interchange dissociative (Id) mechanistic scheme as predicted by us was also strengthened by the large negative entropy of activation.