M. Hofmann et al., 6-VERTEX NIDO-CARBORANE STRUCTURES WITH UNUSUAL CHB BRIDGES OR ENDO-CH HYDROGENS, Journal of organometallic chemistry, 550(1-2), 1998, pp. 331-340
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.