AbstractThe discovery of borospherenes unveiled the capacity of boron to form fullerene-like cage structures. While fullerenes are known to entrap metal atoms to form endohedral metallofullerenes, few metal atoms have been observed to be part of the fullerene cages. Here we report the observation of a class of remarkable metallo-borospherenes, where metal atoms are integral parts of the cage surface. We have produced La₃B₁₈^– and Tb₃B₁₈^– and probed their structures and bonding using photoelectron spectroscopy and theoretical calculations. Global minimum searches revealed that the most stable structures of Ln₃B₁₈^– are hollow cages with D_3h symmetry. The B₁₈-framework in the Ln₃B₁₈^– cages can be viewed as consisting of two triangular B₆ motifs connected by three B₂ units, forming three shared B₁₀ rings which are coordinated to the three Ln atoms on the cage surface. These metallo-borospherenes represent a new class of unusual geometry that has not been observed in chemistry heretofore.