The electronic structure and chemical bonding of B5− and B5 were investigated using anion photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were obtained for B5− and were compared to theoretical calculations performed at various levels of theory. Extensive searches were carried out for the global minimum of B5−, which was found to have a planar C2v structure with a closed-shell ground state (1A1). Excellent agreement was observed between ab initio detachment energies and the experimental spectra, firmly establishing the ground-state structures for both B5− and B5. The chemical bonding in B5− was investigated and compared to that in Al5−. While both B5− and Al5− have a similar C2v planar structure, their π-bonding orbitals are different. In Al5−, a π-bonding orbital was previously observed to delocalize over only the three central atoms in the C2v ground-state structure, whereas a similar π orbital (1b1) was found to completely delocalize over all five atoms in the C2v B5−. This π bonding in B5− makes it more rigid towards butterfly out-of-plane distortions relative to Al5−.
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Zhai et al. (2002) studied this question.