An unusual function of the anion of 1-hydroxyethane-1,1-diphosphonic acid (H4L): Crystal structure of [Ni(Phen)3](H7L2)0.5(H5L2)0.5 ⋅ 2H2O

Crystals of [Ni(Phen)3](H3L)2 ⋅ 2H2O (H4L is 1-hydroxyethane-1,1-diphosphonic acid) have been synthesized, and their structure has been determined by X-ray diffraction analysis. The cationic complexes [Ni(Phen)3]2+, the centrosymmetric dimeric anions H7L2− and H5L23−, and the molecules of crystallization water are the structural units of the crystal. The outer-sphere function of the H4L anions in transition metal compounds and also the presence of differently charged dimeric anions are revealed for the first time. The structural units are joined together by an extended system of hydrogen bonds, including symmetric (or pseudosymmetric) hydrogen bonds in dimeric anions (O⋯O, 2.436 and 2.466 Å). The Ni-N(Phen) bond lengths fall in the range 2.074–2.107 Å. The compound has the structural formula [Ni(Phen)3](H7L2)0.5(H5L2)0.5 ⋅ 2H2O.