ABSTRACT
Figure 2.3 View of the crystal structure of •-B12, showing the six intercluster 2e2c bonds for the central B12 cluster [1]. The γ-B12 structure can be understood as a distorted cubic closed packing of icosahedral B12 clusters as spheres. In this each B12 cluster is connected to six neighboring B12 units by three-center (3c) bonds. The geometrical arrangement is such that these are perpendicular to the threefold symmetry axis within the same close-packed layer, as shown in Fig. 2.2. Then each cluster is furthermore connected to three B12 clusters in the layer above and three clusters in the layer below through two-center (2c) bonds, as shown in Fig. 2.3. 2.2 Development of Boron NanostructuresStocks pioneering research prepared compounds BnHn+4 (where n = 2, 5, 6, or 10) and BnHn+6 (n = 4 or 5) and were labeled “electron
deficient” as they contained too few electrons in order to form pairs in the molecular structure. Containing (2n+4) or (2n+6) atoms, respectively, these boranes have only (2n+2) or (2n+3) valence shell electron pairs instead of (2n+3) or (2n+5) required for 2c links. R. P. Bell and H. C. Longuet-Higgins provided a structural breakthrough in borane structure development by deducing the structure of diborane, B2H6, from its vibrational spectrum. Subsequently, these developments lead to the inception of the 2e3c bond concept and its importance in the borane molecular bonding. The low-temperature X-ray crystallographic studies of BnHn+mby W. N. Lipscomb and analysis of its structure put borane chemistry on a sound basis. Lipscomb’s studies reflected that n boron atoms and m (endo) hydrogen atoms stay on the inner near-spherical surface, whilst the remaining (exo) hydrogen atoms are accommodated at the outer surface, attached to boron atoms by 2e2c B-H bonds. The inner B-H bonding (endo) evolves (2n+m) electrons, which he characterized into four types of electron bond pairs, namely, sBHB and tBBB 2e3c bonds and yBB and xBH 2e2c bonds. 2.3 Conclusion and OutlookIn short, there is an intriguing and potentially significant difference between carbon and boron due to their fundamental difference in valence electrons of individual atomic orbitals. From its basic electron-deficient feature, it is reasonable to assume that a 2D boron sheet is generally a “frustrated” system which does not have enough electrons to fill all electronic orbitals in a chemical bonding that is based on pure sp2 hybridization, and therefore, it is highly probable it consequently does not exhibit some clear preference for a simple structural motif that is similar to graphene or other allotropes of carbon. Hence, from an energetics perspective, there is no driving force motivating boron to form a well-defined structure, and this might explain the difficulty in the synthesis of single-walled boron nanotubes (SWBNTs) or single-layer boron sheets. From the fact that no elemental boron but only compounds containing boron can be found on earth, together with the nature of their electron-deficient chemical bonds, might indicate that these boron structures
