Unifying Chemical Bonding Models for Boranes
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1038-O05-07
Unifying Chemical Bonding Models for Boranes Mao-Hua Du1, Susumu Saito2, and S. B. Zhang3 1 Materials Science & Technology Division and Center for Radiation Detection Materials and Systems, Oak Ridge National Laboratory, PO Box 2008 MS6114, Oak Ridge, TN, 37831 2 Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo, 152-8551, Japan 3 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180 ABSTRACT We demonstrate, based on first-principles calculations, that chemical bonding in deltahedral boron hydrides, BnHn2−, also known as closo boranes, can be understood within the three-center two-electron (3c2e) bonding model in line with other families of boranes. We show that bonding in the triangular lattice of BnHn2− cages can be described by delocalized resonant 3c2e bonding. We also find that the reason for all the BnHn2− to be dianions can be attributed to the reduction of boron coordination number in the deltahedral cage structure from that of boron sheet with triangular lattice. INTRODUCTION The nature of chemical bonding is a fundamental problem in chemistry and physics. Lewis’s seminal work laid out the general concept of bonding based on shared electron pairs between atoms,1 which has been later applied to a wide range of molecules, clusters, and extended systems. The most commonly observed bond in a molecule is the localized two-center two-electron (2c2e) bond, formed by sharing two electrons between two neighboring atoms. However, many molecules may also have multi-center bonds. Boron hydrides, or boranes, belong to this class of molecules by exhibiting a mixture of three-center two-electron (3c2e) and 2c2e bonds. Boron has a rich family of hydrides, such as closo, nido, and arachno boranes.2, 3, 4 The closo boranes (BnHn2−) have cage structures, which are most stable family of boranes. The nido (BnHn+4) and arachno (BnHn+6) boranes have open structures with their skeletons conveniently seen as closo boranes with one and two vertices absent, respectively.5 Carbon is the only other element to also form a complex series of hydrides, i.e., hydrocarbons. However, chemical bonding in boranes is qualitatively different from that in hydrocarbons. For instance, the simplest borane, B2H6, has the same stoichiometry as C2H6 but with two fewer electrons. Because the 2c2e σ bonding mechanism perfectly explains the structure of C2H6, it naturally cannot explain the stability of B2H6. To overcome this problem, the concept of three-center two-electron bonding was developed by Lipscomb and co-workers.6 In the 3c2e picture, the molecule consists of atoms arranged in a triangular network. Each of the three atoms at the vertices of the triangular structure contributes one atomic orbital (AO) such that the three AOs hybridize to form one doubly-occupied bonding molecular orbital (MO) and two empty antibonding MOs. The chemical bonding of B2H6 can thus be explained as having two 3c2e B-H-B bonds and four 2c2e B-H bonds, as illustrated i
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