trans-Difluoridotetrakis(pyridine-κN)chromium(III) perchlorate from synchrotron radiation

The are two independent complex cations in the title salt, [CrF2(C5H5N)4]ClO4, each located on a centre of inversion, as well as an independent perchlorate counter-ion. The complex cations adopt slightly distorted octahedral coordination environments around the CrIII ion, defined by four pyridine (py) N atoms in the equatorial plane and two F− ligands in the axial positions; intramolecular C—H⋯F contacts are noted. The mean Cr—N(py) and Cr—F bond lengths are 2.088 (6) and 1.8559 (10) Å, respectively. The three-dimensional architecture is sustained by hydrogen bonds involving the pyridine C—H groups as donors, and F and O atoms as acceptors.

The are two independent complex cations in the title salt, [CrF 2 (C 5 H 5 N) 4 ]ClO 4 , each located on a centre of inversion, as well as an independent perchlorate counter-ion. The complex cations adopt slightly distorted octahedral coordination environments around the Cr III ion, defined by four pyridine (py) N atoms in the equatorial plane and two F À ligands in the axial positions; intramolecular C-HÁ Á ÁF contacts are noted. The mean Cr-N(py) and Cr-F bond lengths are 2.088 (6) and 1.8559 (10) Å , respectively. The three-dimensional architecture is sustained by hydrogen bonds involving the pyridine C-H groups as donors, and F and O atoms as acceptors.
However, it should be noted that the geometric and conformational assignments based on spectroscopic properties are not always definitive.
In this communication, we describe the structure of trans-[Cr(py) 4 F 2 ]ClO 4 in order to confirm the coordination of four pyridine molecules in equatorial plane and two fluoride ligands in axial positions. Counter anionic species play a very important role in coordination chemistry. This is another example of a trans-[Cr(py) 4 F 2 ] + structure but with a different counter anion.
The structural analysis shows the Cr III complex cation to be coordinated by four nitrogen atoms of four py ligands in the equatorial sites and the two mutually trans fluoride atoms. The Cr1 and Cr2 complex cations are in half occupancy in the asymmetric unit. That is, each molecule is contributing a charge of +0.5. Thus, the salt comprises trans-[Cr(py) 4 F 2 ] + and ClO 4 -. An ellipsoid plot of one independent complex cation and the anion is depicted in Fig. 1.
Atoms Cr1 and Cr2 are located at a crystallographic center of symmetry, so these Cr complex cations have molecular C i symmetry.

1991).
The ClO 4anion remains outside the coordination sphere. The crystal packing is stabilized by hydrogen bonding interactions between the C-H groups of the py ligand and the oxygens of the ClO 4anion, Table 1. As expected, the ClO 4counter ion has slightly distorted tetrahedral geometry due to the influence of hydrogen bonding on the Cl-O bond lengths and the O-Cl-O angles. Consideration of the crystal packing shows that intermolecular C-H···F hydrogen bonds are also present, Table 1.

Experimental
All chemicals were reagent grade materials and used without further purification. The trans-[Cr(py) 4 F 2 ]ClO 4 salt was prepared as described previously (Glerup et al., 1970), and allowed to stand in 0.1 M HClO 4 solution at room temperature for 1-2 days to give very small crystals suitable for X-ray structural analysis.

Refinement
C-bound H-atoms were placed in calculated positions (C-H = 0.95) and were included in the refinement in the riding model approximation with U iso (H) set to 1.2U eq (C).

Special details
Experimental. Since the Pohang Accelerator Laboratory goniostat has only one omega-axis, diffrn_measured_fraction_theta_full is not fully covered as 0.944, especially for the low symmetry such as a triclinic system. As this is an inherent problem, other command and option (such as OMIT) were not helpful to improve the completeness. 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.