Molecular and electronic structure of B10H12L2 compounds, where L is a lewis base

Abstract

The geometrical characteristics of the B10H12L2 complexes of the borane nido-cluster |B10H12| with Lewis base molecules L=NH3, CH3CN, Me2S, Py, which are typical electron pair donors, are analyzed. The complexes are calculated by the MNDO quantum chemical method. Chemical bonding between |B10H12| and L is characterized using, the fragment analysis of molecular orbitals (MOs). It is shown that both occupied and 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 orbitals of the cluster. The electron density is transferred from one L orbital to two unoccupied MOs of |B10H12|, 9a1 and 6b2. There is a large group of HOMOs defined as lone electron pairs of |B10H12| (boron AOs), whose energies change almost symbatically due to bonding to L. The localization of the electron paris by an ab initio calculation in the STO-3G basis set revealed ten three-center bonds and one four-center bond, B(1)-B(2)-B(3)-B(4). The charge transferred from L to |B10H12| is localized mainly on the B(6) and B(9) atoms. The B-L bond is of σ type; a significant θ component is observed for L=CH3CN and Py. It is shown that the simplest two-orbital model of a donor-acceptor bond is inapplicable in the case of the B10H12L2 compounds.