Experimental evidence of orbital order in α-B12 and γ-B28 polymorphs of elemental boron

The electron density of the alpha form of boron has been obtained by multipole refinement against high-resolution, single-crystal x-ray diffraction data measured on a high-quality single crystal at a temperature of 100 K. Topological properties of this density have been used to show that all chemical bonds between B-12 clusters in alpha-B-12 are formed due to one orbital on each boron atom that is oriented perpendicular to the surface of the cluster. It is shown that the same orbital order on B-12 clusters persists in both alpha-B-12 and gamma B-28 polymorphs and in several dodecaboranes, despite the fact that in every case the B-12 clusters participate in entirely different kinds of exocluster bonds. It is likely that the same orbital order of B-12 clusters can explain bonding in other boron polymorphs and boron-rich solids.