Structure and bonding in B6- and B6:: Planarity and antiaromaticity

The electronic structure and chemical bonding of B-6(-) and B-6 were investigated using anion photoelectron spectroscopy and ab initio calculation. Vibrationally resolved photoelectron spectra were obtained for B-6(-) and were compared to calculations performed at various levels of theory. Extensive searches were carried out for the global minimum of B-6(-), which was found to have a planar D-2h structure with a doublet ground state (B-2(2g)). Good agreement was observed between ab initio detachment energies and the experimental spectra, establishing that the ground-state structure of B-6(-) is planar, in contrast to the three-dimensional structures for the valence-isoelectronic Al-6(-) and Al-6 species. The chemical bonding in B-6(-) was interpreted in terms of linear combinations of molecular orbitals of two B-3(-) fragments. The antiaromatic nature of chemical bonding was established for B-6(-) and B-6(2-), based on the analysis of orbital contributions to overall paratropic ring currents.