Tetracarbaboranes: nido structures without bridging hydrogens.

The structures and energetics of the tetracarbaboranes CBH (n = 6 to 13) have been investigated by density functional theory and coupled cluster calculations. In general, the lowest energy structures of the tetracarbaboranes CBH minimize the number of C-C polyhedral edges as well as the degrees of the carbon vertices. For the CBH and CBH systems the lowest energy structures are pyramidal structures having all four carbon atoms located on the base of the pyramid. The lowest energy structure for the 9-vertex CBH system is a capped square antiprism. The frameworks of the lowest energy CBH and CBH structures resemble those of the isoelectronic experimentally known BH and BH structures. However, an experimentally known S adamantane-like 10-vertex structure found in MeCBEt based on a tetracapped octahedron lies only ∼7 kcal mol in energy above the lowest energy structure. The lowest energy structures for the 11- to 13-vertex CBH (n = 11, 12, 13) systems can be derived from an (n + 1)-vertex closo deltahedron by removing a high-degree vertex. At least three of the four carbon atoms are located on edges of the resulting pentagonal or hexagonal open face in the low-energy structures. However, the structures of the experimentally known RCBH (R = Me, Et) obtained from the dimerization of RCBH differ from these low-energy structures. The MeCBH polyhedron has a C chain and two tetragonal faces whereas the EtCBH polyhedron has a hexagonal face with two C units. These structures lie within 2 kcal mol of each other thereby accounting for the fluxional properties of these systems observed by NMR spectroscopy.