Imidazolium trans-diaquadioxalatochromate(III) dihydrate

In the title hydrated molecular salt, (C3H5N2)[Cr(C2O4)2(H2O)2]·2H2O, the complete cation is generated by a crystallographic twofold rotation axis, with one C atom lying on the rotation axis. The complete anion is generated by crystallographic inversion symmetry (CrIII site symmetry -1), to generate a slightly distorted CrO6 octahedron with trans water molecules and chelating oxalate dianions. The oxalate ion is almost planar (r.m.s. deviation = 0.017 Å) and the five-membered chelate ring is a shallow envelope with the metal ion displaced by 0.126 (1) Å from the ligand atoms. The crystal structure features O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds, which link the components into a three-dimensional network.

In the title hydrated molecular salt, (C 3 H 5 N 2 )[Cr(C 2 O 4 ) 2 -(H 2 O) 2 ]Á2H 2 O, the complete cation is generated by a crystallographic twofold rotation axis, with one C atom lying on the rotation axis. The complete anion is generated by crystallographic inversion symmetry (Cr III site symmetry 1), to generate a slightly distorted CrO 6 octahedron with trans water molecules and chelating oxalate dianions. The oxalate ion is almost planar (r.m.s. deviation = 0.017 Å ) and the fivemembered chelate ring is a shallow envelope with the metal ion displaced by 0.126 (1) Å from the ligand atoms. The crystal structure features O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, which link the components into a threedimensional network.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB7157). metal-organic compounds The asymmetric unit of (I) is formed by one-half cation, one half anion and one water molecule of crystallization ( Fig.   1). The Cr +III ion lies on an inversion center and the C and H atoms of C(4)-H(6) groups lie on twofold rotation axis. In the anionic complex, the coordination environment of Cr +III ion involves six oxygen atoms (two from trans water molecules and four from two chelating oxalate dianion) in a slightly distorted octahedral geometry. The main distortion of the CrO 6 octahedron is associated to the reduction from the ideal 90° value of some bond angles [83.11 (5) (2) interactions. As a consequence, the overall hydrogen-bonded scheme can be described as a three-dimensional network.

Experimental
A mixture of imidazole (1 mmol), oxalic acid dihydrate (2 mmol) and Cr(NO 3 ) 3 ·9H 2 O (1 mmol) was dissolved in 40 ml of water. The resulting solution was then stirred for 2 h and allowed to evaporate at room temperature. After two months, violet prisms of (I) were obtained.

Refinement
All non hydrogen atoms were treated anisotropically and the H atoms were refined isotropically. After several cycles of refinement, the hydrogen atoms were found in a difference Fourier map and the H(7) was placed in calculated position with C-H distance of 0.93 Å and constrained to ride on his parent atom [C(3)] with U iso (H)=1.2U eq (C).

Figure 1
A view of the title compound with displacement ellipsoids drawn at the 50% probability level for non-H atoms.