Probing the Structures and Lanthanum–Lanthanum Bonding in La₂Bn– (n = 4–6) Clusters

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, La2Bn- (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 B73- 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.