Probing the Structures and Lanthanum-Lanthanum Bonding in La₂B^- (n = 4-6) Clusters

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, La₂B^- (n = 4-6), using photoelectron spectroscopy and theoretical calculations. Well-resolved photoelectron spectra are obtained and used to verify the global minima of the lanthanide boron clusters. The structures of La₂B₄^- and La₂B₅^- are found to consist of open B₄ and B₅ rings, respectively, around the La₂ dimer equatorially. Theoretical evidence of La-La σ bonding is obtained in La₂B₄^-, whereas the bonding in La₂B₅^- is similar to that of an incomplete inverse sandwich without real La-La bonding. The global minimum of La₂B₆^- is completely different, where one of the La atoms can be viewed as substituting a B atom of the B₇ cluster due to the high electronic stability of the B₇^3- borozene. The resulting lanthaborozene [LaB₆]^3- forms a half-sandwich structure with the second La atom, with evidence of La-La σ bonding. Lanthanide-lanthanide bonds are relatively rare in chemistry. The current work suggests that binary lanthanide boron clusters provide interesting systems to study lanthanide-lanthanide bonding.