Pyridinium cis-diaquabis(oxalato-κ2 O,O′)chromate(III)

The title compound, (C5H6N)[Cr(C2O4)2(H2O)2], contains one protonated pyridine molecule and one [Cr(C2O4)2(H2O)2]− complex anion in the asymmetric unit. The CrIII in the complex anion is coordinated in a distorted octahedral environment by two O atoms from two cis water molecules and four O atoms from two chelating oxalate dianions. The crystal packing is stabilized by intermolecular N—H⋯O(oxalate) and O—H⋯O(oxalate) hydrogen bonds and by π–π stacking interactions (centroid–centroid distance = 3.602 Å) between pyridine rings, thereby building up a three-dimensional network.

The asymmetric unit of the title compound, (C 5 H 6 N)[Cr(C 2 O 4 ) 2 (H 2 O) 2 ] which crstallizes in space group P2 1 /c, is shown in Fig. 1. The Cr III site in the complex anion has a distorted octahedral coordination environment build up by two O atoms (OW1, OW2) from two cis water molecules and four O atoms (O11, O12, O21, O22) from two chelating oxalate dianions. The main geometric parameters of the (C 5 H 6 N) + cation are in agreement with those found in salts with similar cationic entities (Bélombé et al., 2009;Nenwa et al., 2010;Nenwa et al., 2012;Chérif et al., 2011;Chérif, Abdelhak et al., 2012;Chérif, Zid et al., 2012). The bond distances in the complex anion (Table 1)

Experimental
Pyridine (1 mmol, 79.1 mg) and oxalic acid (2 mmol, 260 mg) were added in successive small portions in an aqueous solution (50 ml) of CrCl 3 .6H 2 O (1 mmol, 266.5 mg). The mixture was stirred for 4 h continuously. The final blue-violet solution obtained was left at room temperature and violet crystals suitable for X-ray diffraction were obtained after a few days.

Refinement
The H atoms were positioned geometrically, with C-H, N-H distances of 0.95 and 0.86 Å respectively, and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C,N). The water H atoms were initially located in a difference Fourier map and refined with distance restraints of d(O-H1) = 0.83 (2) with all U iso (H) values refined. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: WinGX (Farrugia, 1999).  The asymmetric unit of the title compound with displacement ellipsoids drawn at the 50% probability level.  Crystal packing of the title compound, showing the components linked via N-H···O and O-H···O hydrogen bonds (dashed lines) forming a three-dimensional network. π-π stacking between the protonated pyridine rings is also observed.

Pyridinium cis-diaquabis(oxalato-κ 2 O,O′)chromate(III)
Crystal data (C 5  Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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