Abstract

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl6- (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl6- (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl6- clusters can be viewed as an M+ cation interacting with an Al-6(2-) dianion. Al-6(2-) was found to possess an O-h ground-state structure, and all of the MAl6- clusters possess a C-3v ground-state structure derived from the O-h Al-6(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C3v ground-state structures for the MAl6- clusters. A detailed molecular orbital (MO) analysis is conducted for Al-6(2-) and compared with Al-3(-) It Was shown that Al-6(2-) can be considered as the fusion of two Al-3(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al-3(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)" MOs of Al-3(-), provides the key bonding interactions for the fusion of the two Al-3(-) into Al-6(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the OhAl62-. It is shown that every face of the Al-6(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al-3(-), and that in fact Al-6(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.