An X-ray diffraction study on the structures of bis- and tris-(ethylenediamine)nickel(II) complexes in solution.

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The structures of nickel(II) complexes with ethylenediamine(en) in solution have been determined by means of X-ray diffraction. The measurements were performed at 25 °C for aqueous ethylenediamine solutions of nickel nitrate, the mole ratios (en/Ni) in the solutions being 3.964, 3.672, 2.071, and 1.892. In the two solutions of the higher en/Ni mole ratio, the tris(ethylenediamine)nickel(II) complex was predominant, whereas in the other solutions the bis(ethylenediamine)nickel(II) complex was contained as the main species. From the analysis of the X-ray scattering data of the solutions, it was shown that in the bis-complex four nitrogen atoms within ethylenediamine and two oxygen atoms within water molecules were octahedrally coordinated to the central nickel(II) atom, the oxygen atoms being assumed to be axially bonded. The Ni–N bond length within the bis-complex was 2.10 Å, which was almost the same as the length of the Ni–O bonds. The nonbonding Ni···C distance was 2.82(2) Å. In the tris-complex the nickel ion was coordinated with six nitrogen atoms and Ni–N and Ni···C distances were determined to be 2.202(4) Å and 2.88(1) Å, respectively. These parameter values obtained in the experiment were compared with the crystallographic data reported.
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