Kinetic studies in aqueous solutions of iron(II)-glycinate, -ethylenediamine, and -malonate complexes

Abstract

The kinetic properties of the complexes formed in aqueous solutions of the Fe²⁺–glycinate (glyO), -ethylenediamine (en) and -malonate (mal) systems have been investigated by measuring the paramagnetic relaxation rate and shift of the CH₂ protons of the ligands over a wide range of complex formation. The formation rate constants (k^f) of the parent complexes are as follows: glycinate, k₁= 5.4 × 10⁶, k₂= 5.0 × 10⁶, and k₃= 1.9 × 10⁶; ethylenediamine, k₁= 7.1 × 10⁷, k₂= 1.8 × 10⁷, and k₃= 2.1 × 10⁶; malonate, k₁= 6.8 × 10⁷ and k₂= 1.3 × 10⁷ dm³ mol^–1 s^–1. Kinetic evidence for the formation of [Fe(gly)]²⁺, [Fe(Hen)]³⁺, and [Fe(Hmal)]⁺ complexes is provided. A ‘carboxyl-displacement’ mechanism is suggested for the interaction between [Fe(glyO)₃]^– and free glyO^– ligand.