ABINITIO MOLECULAR-ORBITAL STUDY OF ELECTRONIC-STRUCTURES OF CLOSO-BORANE ANIONS BNHN2- AND CLOSO-CARBORANES C2BN-2HN

Ab initio molecular orbital (MO) calculations were performed for aa series of closo-boranes BnHn2- (5 less-than-or-equal-to n less-than-or-equal-to 12) and closo-carboranes C2Bn-2Hn (5 less-than-or-equal-to n less-than-or-equal-to 12) with deltahedral skeletons to study the peculiar properties, such as electron deficient and three-center two-electron bonds. By use of the MO's of high quality of double-zeta, the electron density distributions in these compounds were calculated and analyzed in detail through the analytical "spherical charge analysis' by Iwata and electron density contour mapping. It was found that the external BH bonds in both closo-boranes and closo-carboranes have almost similar homopolar bonding character of the typical two-center two-electron bond. The oxidation states of B and C atoms in the series of boranes and carborane isomers were interpreted in terms of the modified oxidation numbers assigned from the spherical charge analysis. Inspection of the contour maps of the deformation electron density on a variety of planes in the deltahedral molecular skeleton reveals that the central planar ring in a three-dimensional cage molecule has the same tendency for the charge distribution and stability as that in planar cyclic conjugated systems.