The Structure Chemistry of Boron-Rich Solids of the Alkali Metals

Abstract

The electronic situation of boron atom frameworks in boron-rich solids is not easy to understand. Attempts to describe such compounds according to known bonding theories such as the Lipscomb or Zintl−Klemm approaches, are not completely unambiguous. To gain further insight and to create a solid experimental basis for the development of a conclusive bonding concept for non-molecular boron-rich compounds, it is essential to investigate metal borides in which the number of electrons transferred from the metal atoms to the boron atom framework appears to be well-defined. Alkali metal borides and carbaborides are such compounds, and it is the aim of this work to provide additional knowledge on their existence and crystal structures. Focussing on the crystal structures of alkali metal borides and carbaborides that are characterised by frameworks of interconnected boron atom polyhedra; it can be shown that there exist convincing geometrical similarities between boron-rich molecules and polyhedral entities in extended networks. This allows for the comparison between the bonding of boron-rich molecules with that of boron-rich solids.