Probing the Structures and Lanthanum-Lanthanum Bonding in La 2 B n - ( n = 4-6) Clusters.

We report an investigation on the structures and chemical bonding in a series of di-lanthanum boron clusters, La 2 B n - ( 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 La 2 B 4 - and La 2 B 5 - are found to consist of open B 4 and B 5 rings, respectively, around the La 2 dimer equatorially. Theoretical evidence of La-La σ bonding is obtained in La 2 B 4 - , whereas the bonding in La 2 B 5 - is similar to that of an incomplete inverse sandwich without real La-La bonding. The global minimum of La 2 B 6 - is completely different, where one of the La atoms can be viewed as substituting a B atom of the B 7 cluster due to the high electronic stability of the B 7 3- borozene. The resulting lanthaborozene [LaB 6 ] 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.