The silver(I)‐catalyzed exchange of coordinated cyanide in hexacyanoferrate(II) by phenylhydrazine in aqueous medium

The [Ag]+-catalyzed exchange of coordinated cyanide in [Fe(CN)6]4− by phenylhydrazine (PhNHNH2) has been studied spectrophotometrically at 488 nm by monitoring increase in the absorbance for the formation of cherry red colored complex [Fe(CN)5PhNHNH2]3−. The other reaction conditions were pH 2.80±,0.02, temperature = 30.0 ± 0.1°C, and ionic strength (I) = 0.02 M (KNO3). The reaction was followed as a function of pH, ionic strength, temperature, [Fe(CN)4−6], [PhNHNH2], [Ag+] by varying one variable at a time. The initial rates were evaluated for each variation using the plane mirror method. The initial rates evaluated as a function of [Fe(CN)4−6] clearly indicate that the initial rate increases with the increase in [Fe(CN)4−6] and finally reaches to a limiting value when [Fe(CN)4−6]/[AgNO3] ⩾ 1000. It indicates the formation of a strong adduct between [Fe(CN)6]4− and AgNO3 prior to the abstraction of CN−. The variation in initial rates with [PhNHNH2] also showed limiting values at [Fe(CN)4−6]/[PhNHNH2] ⩾ 8.30. The complex behavior due to pH and [Ag+] variations on the rate has been explained in detail. The composition of the final reaction product [Fe(CN)5PhNHNH2] formed during the course of reaction has been found to be 1:1 using the mole ratio method. The evaluated values of activation parameters for the catalyzed reaction are Ea = 53.85 kJ mol−1, Δ H≠, = 51.33 kJ mol−1, and Δ S≠ = −134.63 J K−1 mol−1, which suggest an interchange dissociative mechanism. A most plausible mechanistic scheme has been proposed based on the experimental observations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 447–456, 2007

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