Computational Studies of Nido-8-Vertex Boranes, Carboranes, Heteroboranes, and the Lewis Base Adduct nido-B(8)H(10)L.

An extensive investigation of boranes, carboranes, and heteroboranes falling into the nido-8-vertex electron-count class has been carried out using ab initio methods. The results of this study indicate a nido six-membered open face geometry, ni-8, is usually the preferred configuration over a nido five-membered open face geometry, ni-8. In only two systems, B(8)H(9)(3)(-) and OB(7)H(7)(2)(-), is a ni-8 geometry calculated to be of lowest energy. Attempts to test empirical carbon placement rules along with the skeletal bridge and endo-hydrogen location preferences were also evaluated. The results indicate the nido-8-vertex family is not ideally suited for the application of these empirical rules alone. This is probably due to the open face of these clusters not having homogeneous vertexes and/or not being "rigid". The ab initio/IGLO/NMR method was applied to the disputed B(8)H(10).L and C(4)B(4)H(8) systems. The known nido-B(8)H(10).NEt(3) was found to have a ni-8 geometry with a fluxional bridge hydrogen. The calculations confirmed that the known alkylated derivatives of the nido-C(4)B(4)H(8) carboranes have ni-8 configurations in solution. In an investigation of B(8)H(12), a previously unreported isomer of C(2) symmetry was found which high-level G2MP2 calculations indicate is only 1.6 kcal/mol higher in energy than the lowest energy C(s)() symmetry isomer. This C(2) symmetry isomer is likely the higher energy intermediate in the degenerate interconversion of B(8)H(12) 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.

Ab Initio/IGLO/NMR Investigation of nido-C(4)B(7)H(11) Configurations: Structural Assignment of the Three Known Isomers and Reconfirmation of Empirical Carbon Placement Patterns.

The ab initio/IGLO/NMR method has been successfully applied to establish the structures of the three known isomers of nido-C(4)B(7)H(11). The method confirms the previously proposed structure, nido-7,8,9,10-C(4)B(7)H(11), 1a, as one of the three known isomers. Of four candidates considered for the second isomer, one of the previously proposed structures, nido-1,7,8,10-C(4)B(7)H(11), 2b, is selected. Of four candidates considered for the third isomer, structure nido-2,7,9,10-C(4)B(7)H(11), 3b, which had not been previously proposed, is established. The relative order of stability is 1a > 2b > 3b. A comparison of the relative energies of the nine cage structures considered in this study shows that, in complete agreement with previous empirically determined patterns, the most stable structures are those in which the carbons occupy low coordinate sites. This preference is more important than avoiding carbon-carbon connections.