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Three-Dimensional Analogues of Kekule Structures in Borane Chemistry

R. B. King ; Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA

Sažetak

The topology of overlap of the atomic orbitals participating in the requisite delocalization for an aromatic system can be described by a graph, the eigenvalue spectrum of which can be related to molecular orbital energy parameters as determined by Huckel theory. Such aromatic systems can be classified according to the nodality of overlapping atomic orbitals. In particular the three-dimensional deltahedral boranes BnH~- and carboranes Bn _ 2C2Hn (6 ~ n ~ 12) are examples of anodal orbital aromatic systems since the delocalization in their bonding arises from overlap of anodal radial
boron OI' carbon sp hybrids. The concept of Kekule structures for two-dimensional benzenoid hydrocarbons can·be extended to delocalized three-dimensional polyhedral boranes by using three-center B-B-B bonds instead of carbon-carbon double bonds. Such structures for elasa deltahedral boranes BnH~- have three two-center B-B bonds and and n - 2 three-center B-B-B bonds with exactly half of the 2n - 4 faces of deltahedron covered by the three-center B-B-B bonds. Related structures for nido boranes derived from protonation of the hypothetical BnH;;- and arachno boranes derived from
protonation of the hypothetical BnH~- without producing BH2vertices eontain a total of six (two-center B-B + three-center B-H-B bonds) and n - 4 three-center B-B-B bonds for the nido boranes and a total of nine (two-center B-B + three-center B-H-B bonds) and n - 6 three-center B-B-B bonds for the arachno boranes. The feasibility of deltahedral species isoelectronic and isolobal with the neutral BnH!- (e.g., the »boron subhalides« BnX, •• X = Cl, Br, 1) and the tetraanion B"H;;- in addition to the very stable B"H~- can be evaluated by the existence of satisfactory Kekule-type structures with B-B two-center and B-B-B three-center bonds and the degeneracies of the
highest occupied and lowest unoccupied molecular orbitals.

Ključne riječi

Hrčak ID:

176567

URI

https://hrcak.srce.hr/176567

Datum izdavanja:

1.2.1995.

Posjeta: 783 *