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

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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.

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