Kinetics and mechanism of formation of square-planar copper(II) and nickel(II) complexes of ethylenebisbiguanide in aqueous acetate buffer media

Abstract

The kinetics of formation of square-planar CuII and NiII complexes of the quadridentate ligand, ethylenebisbiguanide, have been studied spectrophotometrically in aqueous HOAc−NaOAc buffer, at ionic strength 0.2 mol dm−3, in the 25–35°C temperature range. The observed rate constants for the formation reactions are independent of pH (and of OAc− concentration) in the pH range used (3.6–4.8 for CuII and 5.0–5.8 for NiII) where the product complexes form stoichiometrically, but show first-order dependence on the ligand concentration;i.e. kobs=kf[L]total. At 25°C kf values (dm3 mol−1s−1) are 35.2±0.4 for CuII and (8.4±0.1)×10−3 for NiII. The mechanism of the reactions is discussed.