An X-Ray Diffraction Study on the Structures of Mono(glycinato)zinc(II) and Tris(glycinato)zincate(II) Complexes in Aqueous Solution

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Abstract
The structure of zinc(II) complexes with glycinate ion in solution was investigated by the X-ray diffraction method. The measurements were performed at 20 °C for the solutions with the Cgly⁄Czn mole ratios of 1.49 (solution A) and 5.09 (solution B), where Ci denotes the total concentration of species i. From the analysis of the radial distribution curve of solution A, in which comparable amounts of the hexaaqua- and mono(glycinato)zinc(II) complexes existed, it was found that the mono(glycinato)zinc(II) complex combined with four water molecules at the distance of (2.12±0.01) Å. The Zn–OH2 bond length within the complex was longer than that within the hexaaquazinc(II) ion (2.08 Å). The Zn–O and Zn–N distances, where O and N denote oxygen and nitrogen atoms within the glyeinate ion in the [Zn(gly)(OH2)4]+ complex, were both (2.12±0.02) Å. Thus the mono(glycinato)zinc(II) complex had a regular octahedral structure. The nonbonding Zn···C distance was found to be (2.84±0.02) Å. The X-ray diffraction data of solution B, which contained the [Zn(gly)3]− ion as a predominant species, showed that the tris(glycinato)zincate(II) complex had a regular octahedral structure with the both Zn–O and Zn–N bond distances of (2.12±0.01) Å. The nonbonding Zn···C(COO), Zn···C(CH2), and Zn···O(O=C) lengths were (2.87±0.03) Å, (2.93±0.03) Å, and (4.10±0.04) Å, respectively. The solubility of the bis(glycinato)zinc(II) complex was so low that the structural determination of the complex by the present X-ray diffraction method was not possible.

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