6-VERTEX NIDO-CARBORANE STRUCTURES WITH UNUSUAL CHB BRIDGES OR ENDO-CH HYDROGENS

The experimentally-observed C2B4Et6H2 intermediate, claimed to be the first nido-2-carba-pentaborane(8) derivative, is shown by the ab initi o/GIAO/NMR method actually to be a substituted nido-2,4-dicarba-hexabo rane(8) (10). Both nido-2,4-dicarba-hexaboranes(8) and nido-2,3,5-tric arba-hexaboranes(7) have unsymmetrical endo-CH's which are strongly ti lted towards one of the two neighbouring boron atoms to form partial C HB hydrogen bridges. The energies of nido-2,4-C2B4H8 (10) and nido-2,3 ,5-C3B3H7 are 4.6 and 10.6 kcal mol(-1) higher than the most stable is omers, 2,3-C2B4H8 and 2,3,4-C3B3H7, respectively. The ca. 10 kcal mol( -1) barriers for hydrogen migration in the anions, 1,2- and 2,3-C2B4H7 -, as well as in 1,2-C2B4H8 involve endo-BH transition structures. A C 2B4R6H2 cluster derived from a pentagonal bipyramid by removing a basa l vertex is proposed as another intermediate in the arachno-1-carba-pe ntaborane(10) to closo-1,5-dicarba-pentaborane(5) degradation. (C) 199 8 Elsevier Science S.A.