Probing the Structures and Lanthanum-Lanthanum Bonding in LaB ( = 4-6) Clusters.

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, LaB ( = 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 LaB and LaB are found to consist of open B and B rings, respectively, around the La dimer equatorially. Theoretical evidence of La-La σ bonding is obtained in LaB, whereas the bonding in LaB is similar to that of an incomplete inverse sandwich without real La-La bonding. The global minimum of LaB is completely different, where one of the La atoms can be viewed as substituting a B atom of the B cluster due to the high electronic stability of the B borozene. The resulting lanthaborozene [LaB] 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.