Tetraaquabis(isonicotinamide-κN 1)nickel(II) bis(4-formylbenzoate) dihydrate

The asymmetric unit of the title complex, [Ni(C6H6N2O)2(H2O)4](C8H5O3)2·2H2O, contains one-half of the complex cation with the NiII atom located on an inversion center, a 4-formylbenzoate (FB) counter-anion and an uncoordinated water molecule. The four O atoms in the equatorial plane around the Ni atom form a slightly distorted square-planar arrangement and the slightly distorted octahedral coordination is completed by the two N atoms of the isonicotinamide (INA) ligands at a sligthly longer distance in the axial positions. The dihedral angle between the carboxylate group and the attached benzene ring is 8.14 (11)°, while the pyridine and benzene rings are oriented at a dihedral angle of 3.46 (6)°. In the crystal structure, O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds link the molecules into a three-dimensional network. π–π Contacts between the benzene and pyridine rings [centroid–centroid distance = 3.751 (1) Å] may further stabilize the crystal structure.

The asymmetric unit of the title complex, [Ni(C 6 H 6 N 2 O) 2 -(H 2 O) 4 ](C 8 H 5 O 3 ) 2 Á2H 2 O, contains one-half of the complex cation with the Ni II atom located on an inversion center, a 4formylbenzoate (FB) counter-anion and an uncoordinated water molecule. The four O atoms in the equatorial plane around the Ni atom form a slightly distorted square-planar arrangement and the slightly distorted octahedral coordination is completed by the two N atoms of the isonicotinamide (INA) ligands at a sligthly longer distance in the axial positions. The dihedral angle between the carboxylate group and the attached benzene ring is 8.14 (11) , while the pyridine and benzene rings are oriented at a dihedral angle of 3.46 (6) . In the crystal structure, O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds link the molecules into a three-dimensional network. -Contacts between the benzene and pyridine rings [centroid-centroid distance = 3.751 (1) Å ] may further stabilize the crystal structure.
The title compound is a monomeric complex, with Ni II ion on a centre of symmetry. The structures of some DENA and/or NA complexes of Ni II ion, [Ni(C 8 (Table 2) link the coordinated and uncoordinated water molecules to the FB anion and INA ligand, respectively. The dihedral angle between the carboxyl group (O1/O2/C1) and the benzene ring A (C2-C7) is 8.14 (11)°, while that between rings A and B (N1/C8-C12) is 3.46 (6)°.
In the crystal structure, the O-H···O, N-H···O and C-H···O hydrogen bonds (Table 2) link the molecules into a three-dimensional network, in which they may be effective in the stabilization of the structure. The π-π contact between the benzene and pyridine rings, Cg1-Cg2, [where Cg1 and Cg2 are centroids of the rings A (C2-C7) and B (N1/C8-C12), respectively] may further stabilize the structure, with centroid-centroid distance of 3.751 (1) Å.

S2. Experimental
The title compound was prepared by the reaction of NiSO 4 .6H 2 O (1.31 g, 5 mmol) in H 2 O (25 ml) and INA (1.22 g, 10 mmol) in H 2 O (40 ml) with sodium 4-formylbenzoate (1.72 g, 10 mmol) in H 2 O (50 ml). The mixture was filtered and set aside to crystallize at ambient temperature for several days, giving blue single crystals.

S3. Refinement
Atoms H51, H52, H61, H62, H71 and H72 (for H 2 O), H21 and H22 (for NH 2 ) and H14 (for CH) were located in difference Fourier map and refined isotropically. The remaining H atoms were positioned geometrically with C-H = 0.93 Å, for aromatic H atoms and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C).  The molecular structure of the title molecule with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Primed atoms are generated by the symmetry operator:(′) -x, -y, -z. Dashed lines indicate the hydrogen bonding.

Tetraaquabis(isonicotinamide-κN 1 )nickel(II) bis(4-formylbenzoate) dihydrate
Crystal data [Ni(C 6  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.50 e Å −3 Δρ min = −0.47 e Å −3 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.