Dipotassium tetraaquabis(μ-citrato-κ4 O:O′,O′′,O′′′)nickelate(II) tetrahydrate

The title complex, K2[Ni2(C6H5O7)2(H2O)4]·4H2O, is a dinuclear centrosymmetric anionic octahedral complex, involving citrates as tridentate and bridging ligands, and coordinating water molecules. An extensive network of hydrogen bonds connects the complex anions through the two unique uncoordinating water molecules. The K+ counter cation is surrounded by seven O atoms in the form of an irregular polyhedron and further stabilizes the crystal packing.

The title complex, K 2 [Ni 2 (C 6 H 5 O 7 ) 2 (H 2 O) 4 ]Á4H 2 O, is a dinuclear centrosymmetric anionic octahedral complex, involving citrates as tridentate and bridging ligands, and coordinating water molecules. An extensive network of hydrogen bonds connects the complex anions through the two unique uncoordinating water molecules. The K + counter cation is surrounded by seven O atoms in the form of an irregular polyhedron and further stabilizes the crystal packing.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: KP2456).

Comment
The construction of metal-organic frameworks (MOFs) is an area of intense research activity due to their intriguing structural diversity and potential applications as zeolitic, optoelectronic, magnetic and conducting materials (Chui et al., 1999;Kiang et al., 1999;Kahn & Martinez, 1998;Lin et al., 1999). Depending on the conformation of carbon chains, the functional group of organic ligands and the type of metal ions, a variety of metal coordination polymers with different topological structures, such as one-dimensional chains (Shin et al., 2003), two-dimensional grids (Kondo et al., 2000), three-dimensional porous motifs (Yao et al., 2007) and helical strands (Wu et al., 2003) were observed. In this paper, we report the synthesis of a dimeric nickel(II) citrate complex by self-assembly under hydrothermal conditions.
In the crystal the centrosymmetric structural unit is a dinuclear Ni II anion ( Fig. 1) and the two potassium cations, and crystalline water molecules. The crystallographic unit is a half of the structural unit. The Ni II ion adopts an octahedral coordination mode. One citrate ligand is bound with an hydroxyl and two carboxylate groups to the Ni II ion, whereas one O atom (O6) from a carboxylate group of a symmetry-related citrate ligand occupies another apex, and two water molecules complete the octahedral environment. The Ni-O distances range from 2.0322 (12) Å to 2.0927 (10) Å (Table   1). Neighbouring dimeric complexes are consolidated into a three-dimensional structure by hydrogen bonds (  (Baker et al., 1983). The complex exists as centrosymmetric dimers, which has identical structure with the title complex, but a difference is that the potassium ions and water molecules of crystallization occupy the spaces between the nickel-citrate dimers in the two cases, resulting in the different formation of the geometry of potassium ion and hydrogen bonds.

Experimental
Citric acid monohydrate (0.048 g), NiCl 2 .6H 2 O (0.042 g) and KOH (0.027 g) were dissolved in 6 ml mixed solvent of DMF-H 2 0 (2:1 v/v), which were placed in a small vial. The mixture was heated at 351 K for 3 d and then cooled to room temperature. Green block crystals of the product were collected by filtration and washed with ethanol several times (88% based on Ni). This synthetic route allowed us to obtain a pure phase. Elemental analysis, calculated (%) for title compound: C 20.06, H 3.62; found C 20.35, H 3.44.

Refinement
H atoms were positioned geometrically, with C-H = 0.93 Å, and allowed to ride during subsequent refinement, with U iso (H) = 1.2U eq (C).

Figure 1
The structure of the dimeric complex anion, with the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity.  Dimeric complexes are consolidated into three-dimensional structures by hydrogen bonds. Symmetry code: -x, 1/2 + y, where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.43 e Å −3 Δρ min = −0.55 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.