Kinetics and equilibrium constants of some pentacyanoferrate(II) complexes of nitrogen and sulfur containing heterocycles

Abstract

The kinetics of the substitution reactions of Fe(CN)(5)H(2)O(3-)ion with a series of nitrogen and sulfur containing heterocycles were studied in aqueous media. In the presence of excess ligand, varied over a large range of concentrations, second-order rate constants were calculated at mu = 0.100 M NaClO4. Activation parameters for the formation reactions were found, Delta H* and Delta S*, 28 +/- 6 kJ/mol and 135 +/- 20 J/mol, respectively. The results are interpreted as being consistent with dissociative, SN1 mechanism. The kinetics of formation and dissociation were studied by stopped-flow technique at several temperatures. An investigation of the kinetics of exchange of coordinated heterocycles for 1,3,5-triazine, yielded rate saturation that is typical of a limiting SN1 mechanism. Activation parameters of the limiting first-order specific rate of dissociations were found with Delta H* and Delta S* 53 +/- 2 kJ/mol and 105 +/- 5 J/mol, respectively. From the specific rates of formation and dissociation reactions the equilibrium constants were calculated. (C) 1998 John Wiley & Sons, Inc