Dilead(II) chromium(III) heptafluoride

Single crystals of the title compound, Pb2CrF7, were obtained by solid-state reaction. The monoclinic structure is isotypic with Pb2RhF7 and is built up of CrF6 3− octahedra isolated from each other, inserted in a fluorite-related matrix of PbF6 distorted octahedra, and PbF8 square antiprisms sharing edges and corners. The seventh F atom is ‘independent’, connected only to three Pb atoms within FPb3 triangles, sharing an edge and building an almost planar Pb4F2 unit, so that the formula can alternatively be written as Pb2F(CrF6).

The seventh fluorine atom is "independent", connected only to 3 Pb atoms in FPb 3 triangles, sharing an edge and building an almost planar Pb 4 F 2 unit ( Fig. 2), so that the formula can be alternatively written as Pb 2 F(CrF 6 ). There are some differences observed with the Pb 2 RhF 7 structure-type. The Pb1 atom appears to be six-coordinated in Pb 2 CrF 7 with Pb-F distances ranging from 2.324 (5) to 2.899 (6) Å, there are then 5 next F atoms being between 3.067 (7) and 3.364 (6) Å whereas in Pb 2 RhF 7 , Pb1 looks eightfold coordinated (Pb-F distances between 2.375 and 2.744 Å), with two next F atoms at 3.105 and 3.219 Å. The bond valence calculations show that the first 6 F atoms around Pb1 cannot really satisfy the Pb 2+ charge and that the 5 next F atoms contribute to the overall bond valence ( Table 2). The behaviour of the cell parameters is surprising in the series of the three isostructural compounds. If all cell parameters are logically smaller in Sr 2 RhF 7 (Grosse & Hoppe, 1987) than in Pb 2 RhF 7 , due to the smaller Sr 2+ size, in Pb 2 CrF 7 one observes that only a and b are smaller but that c is much larger, in spite of the Cr 3+ cation being smaller than Rh 3+ (Table 3). Finally, the cell volume of Pb 2 CrF 7 is even slightly larger than that of Pb 2 RhF 7 .
Usually, such fluorinated phases with high A II /M ratio (A II = Ca, Pb, Sr ; M = 3d element or In, Nb, Zr, etc), are found to be related to the fluorite structure adopted by PbF 2 or SrF 2 . The relation is in general easy to establish due to the presence of "independent" F atoms (not bonded to M) which are found to form characteristic FPb 4 tetrahedra. But this is not obvious here (Fig. 3) since no such FPb 4 tetrahedron is observed. Other compounds containing "independent" fluorine atoms coordinated to three cations (Ca or Sr) in a plane are Ca 2 AlF 7 (Domesle & Hoppe, 1980), Sr 5 Zr 3 F 22 (Le Bail, 1996) and Sr 5 (VOF 5 ) 3 F(H 2 O) 3 (Le Bail et al., 2009), the last two being strongly related to the fluorite structure in which the FSr 3 triangles were found to interconnect the fluorite-related blocks. An examination of the Pb coordinates in Pb 2 CrF 7 along the a axis (which is close to the PbF 2 fluorite cell parameter) shows that they alternate at values x~1/4 and 3/4, forming fluorite-related strips corrugating in the ac plane where highly distorted PbF 8 cubes as expected in the fluorite structure were obtained by small displacements of some of the F atoms as evidenced in Fig. 4 . Similar corrugated strips were observed for the Pb 2 ZrF 8 structure (Le Bail & Laval, 1998), where the Zr 4+ cations occupy bicapped trigonal prisms at positions similar to those of Cr 3+ in the title compound. In Pb 2 ZrF 8 , the Pb 2+ lone pair position was suggested by comparison of the Pb coordination with that of Ba in the isostructural α-Ba 2 ZrF 8 structure, observing some strong distorsions, the stereochemichally supplementary materials sup-2 active lone pair repelling clearly some F atoms. The same exercice is not so obvious here, even by comparing Sr 2 RhF 7 with Pb 2 RhF 7 . However, there is a specific pattern of differences in the bond lengths (3 adjacent short Pb-F bonds and 3 adjacent long ones for Pb1; 4 short and 4 long for Pb2) which could be attributed to repulsion effects involving a somehow weak stereochemically active lone pair producing the longer Pb-F distances. The Pb1 lone pair is thus probably oriented towards the barycenter of the (F5-F6-F2) face of the Pb1F 6 octahedron, a similar reasoning applying to the Pb2 lone pair.

Experimental
Solid state reaction between 2PbF 2 and CrF 3 at 773 K for 96 hours in a platinum tube sealed under argon yieded single crystal of the title compound.

Refinement
The highest residual peak and deepest hole in the final difference map were located respectively 0.67 Å and 0.83 Å from the Pb2 atom. Fig. 1. ORTEP-3 view (Farrugia, 1997) of the regular [Cr III F 6 ] 3octahedron and of the "independent" fluoride ion F1 connected to Pb1 and Pb2 completing the Pb 2 F(CrF 6 ) formula (ellipsoids at the 50% probability level). (Brandenburg, 2005) view of the two F1Pb 3 triangles sharing an edge in order to form the planar F 2 Pb 4 unit with Pb1F 6 distorted octahedra and Pb2F 8 square antiprisms. Fig. 3. Diamond (Brandenburg, 2005)

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.
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 Rfactors(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.