Probing the Electronic Structure and Aromaticity of Pentapnictogen Cluster Anions Pn₅⁻(Pn = P, As, Sb, and Bi) Using Photoelectron Spectroscopy and ab Initio Calculations

The electronic structure and chemical bonding of the pentapnictogen cluster anions, Pn5- (Pn = P, As, Sb, and Bi), were investigated using both photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra were obtained for the anions at several photon energies and were analyzed according to the theoretical calculations. The ground state of all the Pn5- species was found to be the aromatic cyclic D5h structure with a C2v low-lying isomer. We found that the C2v isomer gains stability from P5- to Sb5-, consistent with the experimental observation of the coexistence of both isomers in the spectra of Sb5-. The valence molecular orbitals (MOs) of the D5h Pn5- were analyzed and compared to those of the aromatic C5H5- hydrocarbon. The same set of π-MOs is shown to be occupied in the D5h Pn5- and C5H5- species, except that the MO ordering is slightly different. Whereas the three π-MOs in C5H5- all lie above the σ-MOs, the third π orbital (1a2‘ ‘ in Pn5-) lies below the σ-MOs. The stabilization of the π-MO relative to the σ-MOs seems to be common in inorganic aromatic molecules and distinguishes them from the organic analogues.