Kinetic studies in aqueous solutions of cobalt(II)–ethylenediamine, –malonate, and –glycinate complexes

Abstract

The dynamics of equilibria in aqueous solutions of cobalt(II)–ethylenediamine (en), –malonate (mal), and –glycinate (glyO) systems have been investigated by measuring the paramagnetic relaxation rate and shift of the CH₂ ligand protons over a wide range of complex formation. The formation rate constants (dm³ mol^–1 s^–1) of the cobalt parent complexes at 25 °C and I= 1 mol dm^–3 KCl are: k₂^en= 3.0 × 10⁷, k₃^en= 1.1 × 10⁷, k₁^mal= 6.9 × 10⁷, k₂^mal= 1.6 × 10⁷, k₂^glyO= 2.2 × 10⁶, and k₃^glyO= 6.4 × 10⁵. Kinetic evidence is given for the formation of [Co(Hen)]³⁺, [Co(Hmal)]⁺, and [Co(mal)₃]^4– complexes. The ‘carboxyl-displacement’ mechanism was used to interpret the interaction between [Co(glyO)₃]^– and the free glyO^– ligand. The complex-formation kinetics of some complexes of Co²⁺ and Fe²⁺ were compared and the similarities and differences are discussed.