Bis[(1-ammonioethane-1,1-diyl)diphosphonato-κ2 O,O′]diaquanickel(II) nonahydrate

The title compound, [Ni(C2H8NO6P2)2(H2O)2]·9H2O, exhibits a slightly distorted octahedral coordination environment around the NiII atom. It contains two molecules of 1-aminoethylidenediphosphonic acid in the zwitterionic form, coordinated via O atoms from two phosphonate groups and creating two six-membered chelate rings. Two water molecules in cis positions complete the coordination environment of the NiII atom. The title compound contains nine partly disordered solvent water molecules, which create a three-dimensional network of strong O—H⋯O and N—H⋯O hydrogen bonds.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: EZ2208).

Comment
Organic diphosphonic acids are potentially very powerful chelating agents used in metal extractions and are tested by the pharmaceutical industry for use as efficient drugs for preventing calcification and inhibiting bone resorption (Matczak-Jon & Videnova-Adrabinska, 2005). There is evidence that application of transition metal bisphosphonates can improve fixation of cementless metal implants by enhancing the extent of osseointegration (Eberhardt et al., 2005). In this respect a detailed structure-correlated study of the individual properties and the complex-forming driving factors is desired in order to sufficiently understand bisphosphonate physiological activity.
Several structures of Ni II and Zn(II) aminoethylidenediphosphonates have been reported previously (Li et al. 2007). The main difference between these and the title compound is the presence of two water molecules instead of 1,10-phenanthroline in the coordination environment of the transition metal ion (Li et al. 2007).
The asymmetric unit of the title compound contains one molecule of the complex (Fig. 1). Two molecules of 1-aminoethylidenediphosphonic acid chelate the central metal ion via two oxygen atoms from phosphonic groups forming six membered non-planar metallocycles. Two water molecules situated in cis-positions complete the slightly distorted octahedral coordination environment of the Ni II . The Ni-O bond lengths have expected values, which correlate with previously reported related structures (Li et al., 2007). transfer from one of the phosphonic groups to the amino group, as found for all 1-aminodiphosphonic acids where the amino group does not participate in coordination (Dudko et al., 2009).
The crystal structure of the title compound contains nine solvent water molecules, which interact with the two coordinated water molecules and the hydrophilic phosphonic groups. As a result, a 3-D network of mostly strong O-H···O and N-H···O H-bonds has been found in the structure ( Fig. 2; Table 1).

Experimental
Light green crystals of the title compound were obtained from a mixture of 10 ml (10 -2 mol/l) of a water solution of Ni(NO 3 ) 2 with a 20 ml (10 -2 mol/l) solution of 1-aminoethylidenediphosphonic acid. The resultant solution was stored in a dark place for slow evaporation. After the 20 days suitable crystals for X-ray data collection were obtained.

Refinement
The refinement of the structure showed two disordered water molecules. O atoms O22 and O23 were split over two sites with occupancies 0.86/0.14 and 0.81/0.19 respectively. The positions with smaller occupancies were both refined isotropically.
Hydrogen atoms were found from difference map only for sites with greater occupancy of disordered atom. H atoms bonded to N and O were located in a difference map and refined with Uiso(H) = 1.5Ueq(N) and Uiso(H) = 1.2Ueq(O) respectively.
Methyl hydrogens were geometrically constrained and refined using a riding model with C-H = 0.98 Å for CH 3 [Uiso(H) = 1.5Ueq(C)]. Fig. 1. The molecular structure of the title compound showing the atom labelling scheme and 50% probability displacement ellipsoids for the non-hydrogen atoms. Solvent water molecules are omitted for clarity.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.