Kinetics and mechanism of the reaction between bis 4-(2-pyridylazo) resorcinol ferrate(II) complex and cyanide ion

Summary Cyanide ion reacts with Fe(Par)₂]^2–,i.e. Par=4-(2-pyridylazo)resorcinol to form a 113 mixed cyanocomplex. The reaction has been studied spectrophotometrically at 720 nm _max, pH=11.5±0.02, and I=0.1 M (NaClO₄) at 25±0.1°C. The order with respect to cyanide varies from one to two at high and low cyanide concentrations respectively. The rate constants for respective reactions are k₁=(6.1±0.3)×10^–2 M^–1 s^–1, k₂=(12.6±1.0) M^–2 s^–1. The reverse reaction does not occur at a measurable rate even in presence of a large excess of Par. These observations suggest that Fe(Par)₂]^2– forms a mixed FePar(CN)₃]^3– complex in presence of an excess of cyanide ion. The activation parameters for the reaction have been calculated and used to support a three step mechanism consistent with these results. The effect of ionic strength tends further support to the mechanism.