Structure and Bonding in B₆⁻ and B₆:  Planarity and Antiaromaticity

The electronic structure and chemical bonding of B6- and B6 were investigated using anion photoelectron spectroscopy and ab initio calculation. Vibrationally resolved photoelectron spectra were obtained for B6- and were compared to calculations performed at various levels of theory. Extensive searches were carried out for the global minimum of B6-, which was found to have a planar D2h structure with a doublet ground state (2B2g). Good agreement was observed between ab initio detachment energies and the experimental spectra, establishing that the ground-state structure of B6- is planar, in contrast to the three-dimensional structures for the valence-isoelectronic Al6- and Al6 species. The chemical bonding in B6- was interpreted in terms of linear combinations of molecular orbitals of two B3- fragments. The antiaromatic nature of chemical bonding was established for B6- and B62-, based on the analysis of orbital contributions to overall paratropic ring currents.