Molecular Crystal Structure and Electron Density Distribution in the Crystal of Pentaethyl-1,5-dicarba-closo-pentaborane [C2B3(Et)5] at 120 K.
The mol. and crystal structure and electron d. distribution were studied for a single crystal of pentaethyl-1,5-dicarba-closo-pentaborane C2B3(Et)5 by the high-resoln. x-ray diffraction method at 120 K (5696 obsd. reflections, R = 0.056). The single crystal was grown by in situ crystn. with an IR-laser beam producing a molten zone in a Lindeman capillary. In crystal the mol. of the title compd. 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 .ANG., resp., the axial C...C distance is 2.277 .ANG.. An ab initio calcns. of the mol. 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 mol. symmetry and different orientation of the Et groups in mol. due to the crystal packing effects. Deformation electron d. (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 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. Pos. 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 mol. Topol. anal. of the exptl. charge d. distribution has been performed including an anal. of the Laplacian maps, and these data confirmed all important features of the DED maps.
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