Comparison of the formation rate constants of some chromium(II) and copper(II) complexes

Abstract

The equilibrium kinetics in aqueous solutions of the chromium(II)–ethylenediamine (en) and –iminodiacetate (ida) complexes has been studied using a n.m.r. relaxation method. The paramagnetic relaxation rate and shift of the CH₂ protons of the ligands were measured over a wide range of the complex formation. The formation rate constants (dm³ mol^–1 s^–1) of the mono and bis complexes were determined: k₁^en= 6.0 × 10⁹, k₂^en= 3.2 × 10⁸, k₁^ida= 4.9 × 10⁹, k₂^ida= 2.4 × 10⁷. Kinetic evidence is given for the formation of [Cr(Hida)]⁺ and [Cr(ida)₃]^4– complexes. The complex-formation kinetics of some complexes of Cr²⁺ and Cu²⁺ is compared; the similarities and differences are discussed.