Photoelectron Spectroscopy of Size-Selected Bismuth-Boron Clusters: BiB^- (n = 6-8)

Because of its low toxicity, bismuth is considered to be a "green metal" and has received increasing attention in chemistry and materials science. To understand the chemical bonding of bismuth, here we report a joint experimental and theoretical study on a series of bismuth-doped boron clusters, BiB^- (n = 6-8). Well-resolved photoelectron spectra are obtained and are used to understand the structures and bonding of BiB^- in conjunction with theoretical calculations. Global minimum searches find that all three BiB^- clusters have planar structures with the Bi atom bonded to the edge of the planar B moiety via two Bi-B σ bonds as well as π bonding by the 6p orbital. BiB₆^- is found to consist of a double-chain B₆ with a terminal Bi atom. Both BiB₇^- and BiB₈^- are composed of a Bi atom bonded to the planar global minima of the B₇^- and B₈^- clusters. Chemical bonding analyses reveal that BiB₆^- is doubly antiaromatic, whereas BiB₇^- and BiB₈^- are doubly aromatic. In the neutral BiB (n = 6-8) clusters, except BiB₆ which has a planar structure similar to the anion, the global minima of both BiB₇ and BiB₈ are found to be half-sandwich-type structures due to the high stability of the doubly aromatic B₇^3- and B₈^2- molecular wheel ligands.