Peculiar Covalent Bonds inα-Rhombohedral Boron

A peculiar bonding feature of $\alpha$-rhombohedral boron is found in the electron density distribution of the icosahedral ${\mathrm{B}}_{12}$ cluster. The electron density distribution is obtained by using the maximum entropy method with synchrotron radiation powder data. It clearly shows the characteristic two- and three-center covalent bond network which threads through atoms on the cluster surface. Besides, two kinds of covalent bonds in intercluster space are also shown in detail: a three-center trilaterally formed bond among the three clusters and a two-center bond with a remarkable feature of a bent bond between the clusters. The obtained covalent bonding features indicate the cluster-based nature of this material.