Structural Evolution of Boron Clusters upon Copper Doping in CuB_x^- (x = 4-6)

Photoelectron spectroscopy and theoretical calculations are combined to elucidate the structures and chemical bonding of small boron clusters doped with a copper atom, CuB_x^- (x = 4-6). Relatively complex spectral features are observed and are interpreted by comparison with the theoretical results. Predicted global minimum structures of CuB_x^- (x = 4-6) evince that the Cu atom binds to an apex B atom in each cluster and does not significantly alter the planar boron framework of the corresponding B_x^- clusters. Multielectronic transitions (shakeup processes) are observed in all three systems, a manifestation of strong electron correlation effects. Chemical bonding analyses show that the copper atom preferentially binds to the apex sites with the highest electron localization to form a Cu-B covalent bond. The structures and bonding of CuB_x^- (x = 4-6) are compared with those of the bare B_x^- and the Cu₂B_x^- clusters, providing new insights into the structural and electronic evolution of Cu-doped boron clusters and the transition from Cu-B covalent bonding to ionic bonding.