TOPOLOGICAL ANALYSIS OF THE ELECTRON-DENSITY DISTRIBUTION IN THE CRYSTAL OF 8,9,10,12-TETRAFLUORO-O-CARBORANE ON THE BASIS OF THE HIGH-RESOLUTION X-RAY-DIFFRACTION DATA AT 120 K

A topological analysis of the electron density distribution rho(r) in the crystal of 8,9,10,12-tetrafluoro-o-carborane C2B10H8F4 was perform ed using high-resolution low-temperature (120 K) X-ray diffraction dat a (14 606 reflections, R = 0.026) and a multipole model for data refin ement. Molecules in the crystal (space group Pbcn, Z = 4) occupy speci al positions on a 2-fold symmetry axis passing through the middle of a C-C bond and the opposite B-B bond of the octahedral cage. Deformatio n electron density maps as well as maps of the Laplacian of rho(r) sho wed that electron density is essentially delocalized over the surface of the cage and locally depleted in its center. All B-B and B-C bonds in the polyhedron are characterized by significant bending, which is e vident in shifts of their (3,-1) bond critical points from the straigh t lines between bended atoms. Detailed analysis of rho(r) values and i ts Laplacian in the bond critical points revealed some unexpected feat ures in the bonding pattern of the structure studied, namely, the posi tive value of the Laplacian in the homopolar C-C bond and the negative values for the B-C bonds. These data are compared with corresponding ab initio calculations of small deltahedral boranes and carboranes. It is concluded that the electron-withdrawing effect of the fluorine ato ms causes considerable redistribution of the electron density in the m olecule, and, in particular, this is reflected in the shift of rho(r) from the more electron-rich C-C bonds to the B-C bonds.