Probing the\nStructures and Lanthanum–Lanthanum\nBonding in La<sub>2</sub>B<sub><i>n</i></sub><sup>–</sup> (<i>n</i> = 4–6) Clusters

We report an investigation on the structures and chemical\nbonding\nin a series of di-lanthanum boron clusters, La<sub>2</sub>B<sub><i>n</i></sub><sup>–</sup> (<i>n</i> = 4–6),\nusing photoelectron spectroscopy and theoretical calculations. Well-resolved\nphotoelectron spectra are obtained and used to verify the global minima\nof the lanthanide boron clusters. The structures of La<sub>2</sub>B<sub>4</sub><sup>–</sup> and La<sub>2</sub>B<sub>5</sub><sup>–</sup> are found to consist of open B<sub>4</sub> and B<sub>5</sub> rings, respectively, around the La<sub>2</sub> dimer equatorially.\nTheoretical evidence of La–La σ bonding is obtained in\nLa<sub>2</sub>B<sub>4</sub><sup>–</sup>, whereas the bonding\nin La<sub>2</sub>B<sub>5</sub><sup>–</sup> is similar to that\nof an incomplete inverse sandwich without real La–La bonding.\nThe global minimum of La<sub>2</sub>B<sub>6</sub><sup>–</sup> is completely different, where one of the La atoms can be viewed\nas substituting a B atom of the B<sub>7</sub> cluster due to the high\nelectronic stability of the B<sub>7</sub><sup>3–</sup> borozene.\nThe resulting lanthaborozene [LaB<sub>6</sub>]<sup>3–</sup> forms a half-sandwich structure with the second La atom, with evidence\nof La–La σ bonding. Lanthanide–lanthanide bonds\nare relatively rare in chemistry. The current work suggests that binary\nlanthanide boron clusters provide interesting systems to study lanthanide–lanthanide\nbonding.