Equilibria in aqueous solutions of some chromium(2+) complexes

Abstract

The stepwise formation constants of the complexes formed in the Cr²⁺–ethylenediamine (en), –malonate (mal), –glycinate (glyO), –β-alaninate (alaO), –iminodiacetate (imda), –nitrilotriacetate (nta), and –ethylenediaminetetra-acetate (edta) systems in aqueous 1 mol dm^–3 KCI at 25 °C, determined pH-metrically, are as follows: log K₁^en= 5.48, log K₂^en= 4.15, log K₁^mal= 3.57, log K₂^mal= 1.92, log K₁^glyo= 4.21, log K₂^glyo= 3.06, log K₁^alao= 3.89, log K₁^imda= 5.01, log K₂^imda= 3.17, log K₁^nta= 6.52, log K₂^nta= 3.14, log K₁^edta= 12.70. The logarithm of the equilibrium constant for the Cr²⁺= Cr(OH)⁺+ H⁺ process is –5.30; the formation of a protonaied complex was detected in the Cr²⁺–malonate and –ethylenediaminetetra-acetate systems and the formation of a binuclear [Cr₂(nta)]⁺ complex in the Cr²⁺–nitrilotriacetate system. The log (K₁/K₂) values for the Cr²⁺ and Cu²⁺ complexes are roughly the same in the case of bidentate ligands, but they are much lower for Cr²⁺ than for Cu²⁺ in the case of multidentate ligands. The ratios of ‘excess’ stabilities of the Cr²⁺ and Cu²⁺ complexes are also roughly the same (0.54) in the case of bidentate ligands, but no excess stability (even destabilization) of the Cr²⁺ complexes was found in the case of the multidentate ligands. These phenomena are interpreted in terms of the difference in ionic radii of the two metal ions.