o-, m-, and p-carboranes and their anions:: Ab initio calculations of structures, electron affinities, and acidities

Ab initio geometry optimizations have been performed for o-, m-, and p-C2B10H12 carborane cages and their mono- and dianions at the Hartree-Fock 3-21G*, 6-31G*, and 6-31G** levels. The para-compound is the most stable with a calculated Delta H(para-meta) value of -10 kJ/mol and a calculated Delta G(975K)(para-meta) value of -3 kJ/mol. These small calculated energy differences disagree with the experimental heats of formation, which suggest Delta H to be ca. -65 kJ/mol, but they agree with equilibrium gas mixture measurements at 975 K, which suggest a Delta G value of ca. -1 kJ/mol. Electron distributions, electron affinities, and acidic properties of the carboranes are presented. The structures of the mono- and dianions were determined by optimization, starting from the three neutral carborane cages; we find three energy minima for the C2B10H122- ion. The lowest-energy dianion is a nido-structure topped by a six-membered ring with the two C atoms in the meta position. The captured electrons become distributed over all the H atoms.