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Structure and bonding in MPb5(-) (M = Cu, Ag, and Au): a combined investigation by theoretical calculations and photoelectron imaging spectroscopy.

Bimetallic clusters of MPb5(-) (M = Cu, Ag, and Au) have been studied using density functional theory and photoelectron imaging spectroscopy. These anionic clusters and their neutrals were determined to be a Pb5 trigonal bipyramid with the coinage metal atom on its triangular facet. This structure of each MPb5(-) or MPb5 was found to be more than 0.5 eV lower than other structural candidates and that of each MPb5(-) has a HOMO-LUMO gap of larger than 1.2 eV. The chemical bonding between M and Pb5 in MPb5(-) was dominantly attributed to the interaction between the outer s orbital of M and the lowest unoccupied molecular orbital (LUMO) of Pb5. The inner d orbitals of M and the occupied orbitals of Pb5 unit only make a little contribution. The different bonding behaviors of Cu, Ag, and Au, which are noticeable in many other species, have little effect on the Pb5 counterpart in MPb5(-), indicating Pb5 unit acts partially like a large artificial atom. Additionally, photoelectron spectra of MPb5(-) (M = Cu, Ag, and Au) provide good experimental data to evaluate different theoretical approaches dealing with relativistic effects in clusters containing heavy atoms.