Bis(3-aminopyrazine-2-carboxylato-κ2 N 1,O)diaquanickel(II) dihydrate

In the title compound, [Ni(C5H4N3O2)2(H2O)2]·2H2O, the NiII ion lies on an inversion center and is coordinated in an slightly distorted octahedral environment by two N,O-chelating 3-aminopyrazine-2-carboxylate (APZC) ligands in the equatorial plane and two trans-axial aqua ligands. In the crystal, O—H⋯O, N—H⋯O and O—H⋯N hydrogen bonds involving the solvent water molecules, aqua and APZC ligands form layers parallel to (010). These layers are linked further via O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds involving the axial aqua ligands, amino groups and the carboxylate groups of the APZC ligands, forming a three-dimensional network.

In continuation of our search to enrich the variety of such kinds of hybrid compounds and to investigate the influence of hydrogen bonds on the structural features (Bouacida et al., 2007(Bouacida et al., , 2009, we report here the synthesis and crystal structure of the title compound, (I), as a extention of our earlier work on N,O chelated ligands (Bouchene et al.2013) which can be involved in covalent interactions in metal coordination chemistry.
The asymmetric unit of (I) consists of one-half of the molecule, with the other half generated by a crystallographic inversion center. The molecular structure is shown in Fig. 1

Experimental
Nickel dichloride hexahydrate (0.2 mmol) and 3-aminopyrazine-2-carboxylic acid (0.02 mmol) were dissolved in acidified water with concentrated hydrogen chloride acid (37%). Light green crystals, suitable for X-ray diffraction study, were obtained from evaporation of obtained solution for three days.

Refinement
The H atoms bonded to C and N were located in differnce Fourier maps but subsequently introduced in calculated positions and treated as riding on their parent atoms (C or N) with C-H = 0.93 Å and N-H = 0.86 Å with U iso (H) = 1.2U eq (C or N). Atoms H1W and H2W were located in a difference Fourier map and refined isotropically wirh U iso (H) = 1.5U eq (O).

Figure 1
The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary radii. Symmetry code: (i)-x+1, -y+1, -z+2.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.2428 (