Abstract

The electronic structure and chemical bonding of the pentapnictogen cluster anions, Pn(5)(-) (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 Pn(5)(-) species was found to be the aromatic cyclic D-5h structure with a C-2nu low-lying isomer. We found that the C-2nu isomer gains stability from P-5(-) to Sb-5(-), consistent with the experimental observation of the coexistence of both isomers in the spectra of Sb-5(-). The valence molecular orbitals (MOs) of the D-5h Pn(5)(-) were analyzed and compared to those of the aromatic C5H5- hydrocarbon. The same set of pi-MOs is shown to be occupied in the D-5h Pn(5)(-) and C5H5- species, except that the MO ordering is slightly different. Whereas the three pi-MOs in C5H5- all lie above the sigma-MOs, the third pi orbital (1a(2)" in Pn(5)(-)) lies below the sigma-MOs. The stabilization of the pi-MO relative to the sigma-MOs seems to be common in inorganic aromatic molecules and distinguishes them from the organic analogues.