FUSED AND LINKED DELTAHEDRAL CLUSTERS IN THE CHEMISTRY OF THE GROUP 13 ELEMENTS

The electronic requirements associated with the linking and fusion via vertex, edge, and face of main group deltahedra are examined by using molecular orbital and tight-binding ideas. The focus is on the chemistry of borides and gallides. For the vertex-linked case it is shown how the traditional ideas of a mixture of multicentered intradeltahedral bonding and localized interdeltahedral bonding naturally fall out of an energy band model. The electron-counting rules for fusion turn out to be considerably more complex than those previously developed for their transition-metal analogues. Especially the interactions between orbitals on atoms not formally included in the fusion process need to be considered. As a result the electron-counting rules are often dependent upon the identity of the deltahedron itself and whether is of the closo, nido, or arachno type. The rules are used to organize the structures of a number of solid-state borides and gallides. A new rule is discovered concerning the electron count per group 13 atom found in extended solid-state structures. It appears that no deltahedral units are found in solids where the number of electrons per deltahedral boron or gallium atom exceeds 3.5. In all materials where the electron count is higher, structures containing six-membered rings or other open structures are found. The result is very different from that found in molecular boranes, where no such restriction occurs. The difference is attributed to the greater structural versatility of the solid-state. Some of the electronic reasons behind the difference between boron and gallium structural chemistry are explored. © 1990, American Chemical Society. All rights reserved.