Structure and Bonding of gamma-B-28: Is the High Pressure Form of Elemental Boron Ionic?

• The recently characterized crystal structure of metastable gamma-B-28 is analyzed from a crystal chemical point of view, and the electron requirement of its building units and that of their linkage is determined. The structure consists of unique B-2 dumbbells and B-12 icosahedra, which are connected through two-center and three-center, two-electron bonds. The different bonding motifs are ascertained by theoretical calculations of difference charge distributions. Chemical bonding in high pressure gamma-B-28 bears great resemblance to alpha-B-12 which is the simplest boron modification. The previously made description of gamma-B-28 as ionic in terms of (B2)(delta+) and (B-12)(delta-) is not supported.

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