COMPUTATIONAL STUDIES OF NIDO-8-VERTEX BORANES, CARBORANES, HETEROBORANES, AND THE LEWIS BASE ADDUCT NIDO-B8H10L

An extensive investigation of boranes, carboranes, and heteroboranes f alling into the nido-8-vertex electron-count class has been carried ou t using ab initio methods. The results of this study indicate a nido s ix-membered open face geometry, ni-8[VI], is usually the preferred con figuration over a nido five-membered open face geometry, ni-8[V]. In o nly two systems, B8H93- and OB7H72-, is a ni-8[V] geometry calculated to be of lowest energy. Attempts to test empirical carbon placement ru les along with the skeletal bridge and endo-hydrogen location preferen ces were also evaluated. The results indicate the nido-8-vertex family is not ideally suited for the application of these empirical rules al one. This is probably due to the open face of these clusters not havin g homogeneous vertexes and/or not being ''rigid''. The ab initio/IGLO/ NMR method was applied to the disputed B8H10. L and C4B4H8 systems. Th e known nido-B8H10. NEt8 was found to have a ni-8[VI] geometry with a fluxional bridge hydrogen. The calculations confirmed that the known a lkylated derivatives of the nido-C4B4H8 carboranes have ni-8[VI] confi gurations in solution. In an investigation of B8H12, a previously unre ported isomer of C-2 symmetry was found which high-level G2MP2 calcula tions indicate is only 1.6 kcal/mol higher in energy than the lowest e nergy C-2 symmetry isomer. This C-2 symmetry isomer is likely the high er energy intermediate in the degenerate interconversion of B8H12 into its mirror image. The transition state for the conversion of the C-s to the C-2 symmetry isomer has C-1 symmetry with a barrier of 2.1 kcal /mol at the MP2/6-31G level of ab initio theory.