Kinetics of substitution and oxidation reactions of the iron(II)-2,3,5,6-tetrakis (2-pyridyl)pyrazine-cyanide system: Observation of rare zero-order kinetics

The kinetics of nucleophilic substitution by cyanide ion at the bis-(2,3,5,6-tetrakis (2-pyridyl)pyrazine)iron(II) cation, [Fe(tppz)(2)](2+) have been measured, and the crystal structure of the product [Fe(tppz)-(CN)(3)](-) anion is reported. The kinetics of oxidation of bath [Fe(tppz)(2)](2+) and [Fe(tppz)(CN)(3)](-) by the peroxodisulfate anion have been studied. While the mechanism for the latter reaction follows a typical second-order-rate law, with activation enthalpy and entropy of 62+/-1 kJ mol(-1) and -82 +/- 3 J K-1 mol(-1), respectively, the oxidation of the [Fe(tppz)(2)](2+) cation obeys a remarkable overall first-order-rate law, that is zero-order in metal complex. The mechanism is interpreted in terms of rate-determining thermal fissure of peroxodisulfate into two sulfate radicals, with subsequent radical attack on the tppz ligand of the complex.