Kinetics and mechanism of replacements in pentacyano(ligand)ferrate(II)ions. An attempt to distinguish between the D and Id mechanisms

Abstract

The kinetics of replacements of 3-cyanopyridine (3CN-py) by pyridine (py) in [Fe(CN)₅(3CN-py)]^3– at 25 °C, and of nitrosobenzene by [CN]^– in [Fe(CN)₅(ONPh)]^3– at 50 °C, have been studied in solutions containing variouswater concentrations achieved by the addition of sorbitol, glucose, sucrose, glycerol, and ethylene glycol. The observed rate constants pertain to a 200-fold excess of the entering ligand, and a 10-fold excess of the leaving ligand, over the concentration of the starting complexes (5 × 10^–5 mol dm^–3). Under the reaction conditions used, sorbitol, glucose, sucrose, and even glycerol do not enter the complexes to any significant degree, but, it seems, ethylene glycol does. A large and analogous reduction in rate with decreasing water concentration occurs in both replacement reactions, which suggests that this is a general effect. The results are interpreted in terms of an I_d mechanism via a [Fe(CN)₅(OH₂)]^3– reaction intermediate in aqueous solutions.