Collect. Czech. Chem. Commun. 2002, 67, 751-768

Strained Configurations in Three-Dimensional Analogues of Kekulé-Type Structures for Deltahedral Boranes

R. Bruce King

Department of Chemistry, University of Georgia, Athens, Georgia 30602, U.S.A.


Localized structures analogous to the Kekulé structures for benzenoid hydrocarbons can be constructed for the deltahedral boranes BnHn2-. These localized structures contain exactly three two-center two-electron (2c-2e) B-B bonds and n - 2 three-center two-electron (3c-2e) B-B-B bonds. The number of equivalent such Kekulé-type structures corresponds to the index of the symmetry group of the Kekulé structure, K, in the symmetry group, D, of the deltahedron. Three-dimensional Kekulé-type structures with the following configurations exhibit excessive strain and are therefore unfavorable: (i) structures having one or more pairs of boron atoms connected simultaneously by a 2c-2e B-B bond and a 3c-2e B-B-B bond (violation of the O'Neill-Wade restrictions); (ii) structures in which the three 2c-2e B-B bonds are excessively concentrated occupying only three or four vertices (the undesirable ∆, U, and Y configurations). Computations by Lipscomb and coworkers with partial retention of diatomic differential overlap (PRDDO) suggest that wide distribution of the three 2c-2e B-B bonds throughout the deltahedron and a minimum number of empty faces are more important than maximum symmetry in leading to the most favorable Kekulé-type structure.

Keywords: Boranes; Aromaticity; Boron clusters; Symmetry; Topology.

References: 35 live references.