MOLECULAR AND ELECTRONIC-STRUCTURE OF B(10)H(12)L(2) COMPOUNDS, WHEREL IS A LEWIS BASE

The geometrical characteristics of the B(10)H(12)L(2) complexes of the borane nido-cluster \B10H12\ with Lewis base molecules L = NH3, CH3CN , Me(2)S, Py, which are typical electron pair donors, are analyzed The complexes are calculated by the MNDO quantum chemical method. Chemica l bonding between \B10H12\ and L is characterized using the fragment a nalysis of molecular orbitals (MOs). It is shown that both occupied an d unoccupied MOs of the \B10H12\ fragment make important contributions to the bonding. The higher MOs of the system as well as its lower MOs with orbital energies up to 20 eV and higher involve the unoccupied o rbitals of the cluster. The electron density is transferred from one L orbital to two unoccupied MOs of \B10H12\, 9a(1) and 6b(2). There is a large group of HOMOs defined as lone electron pairs of \B10H12\ (bor on AOs), whose energies change almost symbatically due to bonding to L . The localization of the electron pairs by an ab initio calculation i n the STO-3G basis set revealed ten three-center bonds and one four-ce nter bond, B(1)-B(2)-B(3)-B(4). The charge transferred from L to \B10H 12\ is localized mainly on the B(6) and B(9) atoms. The B-L bond is of a type; a significant pi component is observed for L = CH3CN and Py. If is shown that the simplest two-orbital model of a donor-acceptor bo nd is inapplicable in the case of the B(10)H(12)L(2) compounds.