Comprehensive analysis of chemical bonding in boron clusters

Abstract

We present a comprehensive analysis of chemical bonding in pure boron clusters. It is now established in joint experimental and theoretical studies that pure boron clusters are planar or quasi-planar at least up to twenty atoms. Their planarity or quasi-planarity was usually discussed in terms of π-delocalization or π-aromaticity. In the current article, we demonstrated that one cannot ignore σ-electrons and that the presence of two-center two-electron (2c-2e) peripheral B - B bonds together with the globally delocalized σ-electrons must be taken into consideration when the shape of pure boron cluster is discussed. The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n + 2 (or 4n) electron counting rule for either π- or σ-electrons in the planar structures. We showed that pure boron clusters could have double (σ- and π-) aromaticity (B3-, B4, B5+, B62+, B7-, B 8, B82-, B9-, B 10, B11+, B12, and B 13+), double (σ- and π-) antiaromaticity (B 62-, B15), or conflicting aromaticity (B 5-,σ-antiaromatic and π-aromatic and B 14, σ-aromatic and π-antiaromatic). Appropriate geometric fit is also an essential factor, which determines the shape of the most stable structures. In all the boron clusters considered here, the peripheral atoms form planar cycles. Peripheral 2c-2e B - B bonds are built up from s to p hybrid atomic orbitals and this enforces the planarity of the cycle. If the given number of central atoms (1, 2, 3, or 4) can perfectly fit the central cavity then the overall structure is planar. Otherwise, central atoms come out of the plane of the cycle and the overall structure is quasi-planar. © 2006 Wiley Periodicals, Inc.