American Chemical Society
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Real-Space Indicators for Chemical Bonding. Experimental and Theoretical Electron Density Studies of Four Deltahedral Boranes

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journal contribution
posted on 2011-01-03, 00:00 authored by Stefan Mebs, Roman Kalinowski, Simon Grabowsky, Diana Förster, Rainer Kickbusch, Eugen Justus, Wolfgang Morgenroth, Carsten Paulmann, Peter Luger, Detlef Gabel, Dieter Lentz
In an approach combining high-resolution X-ray diffraction at low temperatures with density functional theory calculations, two closo-borates, B12H122− (1) and B10H102− (2), and two arachno-boranes, B10H12L2 [L = amine (3) or acetonitrile (4)], were analyzed by means of the atoms-in-molecules (AIM) theory and electron localizability indicator (ELI-D). The two-electron three-center (2e3c) bonds of the borane cages are investigated with the focus on real-space indicators for chemical bonding and electron delocalization. In compound 2, only two of the three expected bond critical points (bcp’s) are found. However, a weakly populated ELI-D basin is found for this pair of adjacent B atoms and the delocalization index and the Source contributions are on the same order of magnitude as those for the other pairs. The opposite situation is found in the arachno-boranes, where no ELI-D basins are found for two types of B−B pairs, which, in turn, exhibit a bcp. However, again the delocalization index is on the same order of magnitude for this bonding interaction. The results show that an unambiguous real-space criterion for chemical bonding is not given yet for this class of compounds. The arachno-boranes carry a special B−B bond, which is the edge of the crown-shaped molecule. This bond is very long and extremely curved inward the B−B−B ring. Nevertheless, the corresponding bond ellipticity is quite small and the ELI-D value at the attractor position of the disynaptic valence basin is remarkably larger than those for all other B−B valence basins. Furthermore, the value of the ED is large in relation to the B−B bond length, so that only this bond type does not follow a linear relationship of the ED value at the bcp versus B−B bond distances, which is found for all other B−B bcp’s. The results indicate that both 2e2c and 2e3c bonding play a distinct role in borane chemistry.