X-ray absorption study of copper(II)-glycinate complexes in aqueous solution

The structures of the mono-, bis-, and tris(glycinato)copper(II) complexes in aqueous solution have been determined by X-ray absorption spectroscopy. Four solutions with different complex ratios have been examined, and the species concentrations have been determined on the basis of complex stability constants. An advanced data analysis including multiple-scattering effects and multielectron excitation processes produced quantitative information on the Cu(II)−glycinate complexes present in aqueous solution. The structure of the bis(glycinato)copper(II) complex, which has a low solubility in water, has been determined for the first time. It has been found to have a distorted octahedral geometry with two bidentate glycine ligands coordinating to the Cu2+ ion in the equatorial plane and with the axial sites occupied by two additional water molecules at 2.40 ± 0.06 Å. Analysis of the X-ray absorption data allowed a detailed description of the structures of the mono- and tris(glycinato)copper(II) species. The former complex has an axially elongated octahedral structure with a glycine bidentate ligand and two water molecules placed at the equatorial and two water molecules at the axial positions (2.44 ± 0.08 Å). Experimental data for the latter complex were also explained in terms of a distorted octahedral model with two glycine molecules in the equatorial plane and with the amino nitrogen of the third glycine coordinating to the Cu2+ ion at the axial site at a distance of 2.33 ± 0.05 Å. This result conflicts with the regular octahedral geometry previously determined by XRD investigations. The axial bonds of the three complexes are significatively longer than the average within the hexaaquocopper(II) ions. This finding indicates that the axial bonds are lengthened upon the formation of the Cu−glycine complexes and the Cu(II)−water interaction at the axial site is weakened.