Hexaaquanickel(II) bis[triaqua-μ3-oxalato-di-μ-oxalato-bariumchromate(III)] tetrahydrate

The structure of the title compound is made up of corrugated anionic layers of formula [BaCr(C2O4)3(H2O)3]n n– that leave voids accommodating the charge-compensating cations, [Ni(H2O)6]2+ (point group symmetry ), as well as the water molecules of crystallization.

] 2+ (point group symmetry 1), as well as the water molecules of crystallization. The anionic layers are built from the connection of barium and chromium atoms through bridging oxalate ligands. The Cr III atom is hexacoordinated by O atoms of three oxalate ligands while the Ba II atom is tenfold coordinated by three O atoms of water molecules and seven O atoms of four oxalate ligands. Each Ni II atom sits on an inversion center and is coordinated by six water molecules. One of the uncoordinated water molecules is disordered over two sites, with a refined occupancy ratio of 0.51 (5):0.49 (5). In the crystal, extensive O-HÁ Á ÁO hydrogen-bonding interactions link the anionic layers, the charge-balancing cations as well as the water molecules of crystallization into a three-dimensional supramolecular network.

Chemical context
Over the past three decades, tris(oxalato)metalate(III) complex anions, [M(C 2 O 4 ) 3 ] 3-, have been extensively used for the design of many compounds with fascinating physical properties (Zhong et al., 1990;Bé nard et al., 2001;Coronado et al., 2008;Pardo et al., 2011;Martin et al., 2017;Tsobnang et al., 2019;O " kawa et al., 2020). One of the main reasons for that is the ability of these anions to act like ligands towards a variety of metallic cations and to build a diversity of extended structures in which neighboring metallic ions are linked through bridging oxalate ligands. From the synthetic point of view, the tris(oxalato)chromate(III) anion, [Cr(C 2 O 4 ) 3 ] 3or [Cr(ox) 3 ] 3-, is most attractive because of its stability and inertness toward ligand substitution. As a source of this anion, the polymeric complex salt {Ba 6 (H 2 O) 17 [Cr(C 2 O 4 ) 3 ] 4 }Á7H 2 O (Bé lombé et al., 2003) offers the possibility of easily replacing, in the reaction medium and under daylight, the Ba 2+ ions by other cations, provided the latter are brought into that medium as their sulfates. Since Ba 2+ has a flexible coordination sphere with coordination numbers ranging from three to twelve (Hancock et al., 2004)

Figure 2
Connection of [Cr(C 2 O 4 ) 3 ] 3units with Ba 2+ cations into a ladder-like chain. Barium-coordinating water molecules have been omitted for clarity.

Figure 1
The components of the asymmetric unit of (I), showing the atomnumbering scheme and displacement ellipsoids at the 50% probability level.

Figure 3
Three adjacent ladder-like chains connected through Ba 2 O 2 units into a corrugated layer, viewed in the (101) plane (a) and along [010] (b). Barium-coordinating water molecules have been omitted for clarity.

Supramolecular features
In the crystal, extensive O-HÁ Á ÁO hydrogen-bonding interactions of medium-to-weak strength are observed (Table 2), with all the water molecules acting as hydrogen-bond donors. The water molecules of crystallization also act as hydrogen-bond acceptors, as well as all of the oxalate O atoms except O12, O14 and O18. Two barium-coordinating water molecules (O1 and O3) behave as hydrogen-bond donors toward both components of the disordered lattice water molecule (O20A and O20B) via three-center bonds, O1-H1BÁ Á Á(O20A,O20B) and O3-H3BÁ Á Á(O20A,O20B). The cationic complex, [Ni(H 2 O) 6 ] 2+ , functions as a hydrogen-bond donor group towards one barium-coordinating water molecule (O3), one water molecule of crystallization (O19) and four oxalate O atoms, viz. O9 vi , O13 vi , O11 iv and O17 iv [symmetry codes refer to Table 2]. Together, these interactions lead to a threedimensional supramolecular network structure.

Figure 4
Packing of the crystal structure of (I) in a view along [010], showing corrugated layers interleaved by [Ni(H 2 O) 6 ] 2+ complex cations and water molecules of crystallization. Barium-coordinating water molecules have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were located in difference-Fourier maps and refined with O-H and HÁ Á ÁH distance restraints of 0.88 (1) and 1.37 (2) Å , respectively, and with U iso (H) = 1.5U eq (O). One lattice water molecule was refined as being disordered over two positions (O20A and O20B), with the occupancy ratio refined to 0.51 (5) (Farrugia, 2012) and DIAMOND (Brandenburg & Putz, 2018); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010).

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.