Molecular Crystal Structure and Electron Density Distribution in the Crystal of Pentaethyl-1,5-dicarba-closo-pentaborane [C2B3(Et)5] at 120 K

Abstract

The molecular and crystal structure and electron density distribution were studied for a single crystal of pentaethyl-1,5-dicarba-closo-pentaborane C2B3(Et)5 by the high-resolution X-ray diffraction method at 120 K (5696 observed reflections, R = 0.056). The single crystal was grown by in situ crystallization with an IR-laser beam producing a molten zone in a Lindeman capillary. In crystal the molecule of the title compound has local C1-symmetry with different orientations of the Et-groups with respect to the central trigonal-bipyramidal C2B3 fragment, mean B−C and B−B distances are 1.571 and 1.876 Å, respectively, the axial C...C distance is 2.277 Å. An ab initio calculations of the molecular structure indicated that rotation barriers of the Et groups with respect to the central frame are very small, that may be the reason of the low molecular symmetry and diffrerent orientation of the Et groups in molecule due to the crystal packing effects. Deformation electron density (DED) maps obtained via a conventional “X−X” method, and using a multipole refinement procedure, showed charge accumulation in the B−C bonds of the central frame, and these bonds were found to be essentially bent outward of the C2B3 cage. On the contrary, no charge accumulation was detected in the B−B bonds, thus indicating the absence of the direct B−B interactions. Positive and delocalized DED was also found in all B−C−B triangle faces of the central cage, that indicates the contribution of the multicenter bonding in the electronic structure of this formally electron-precise molecule. Topological analysis of the experimental charge density distribution has been performed including an analysis of the Laplacian maps, and these data confirmed all important features of the DED maps.