Hexagonal bipyramidal [Ta(2)B(6)](-/0) clusters: B(6) rings as structural motifs

It has been a long-sought goal in cluster science to discover stable atomic clusters as building blocks for cluster-assembled nanomaterials, as exemplified by the fullerenes and their subsequent bulk syntheses.1, 2 Clusters have also been considered as models to understand bulk properties, providing a bridge between molecular and solid-state chemistry.3 Because of its electron deficiency, boron is an interesting element with unusual polymorphism. While bulk boron is known to be dominated by the three-dimensional (3D) B12 icosahedral motifs,4 new forms of elemental boron are continuing to be discovered.5 In contrast to the 3D cages commonly found in bulk boron, in the gas phase two-dimensional (2D) boron clusters are prevalent.6-8 The unusual planar boron clusters have been suggested as potential new bulking blocks or ligands in chemistry.6a Herein we report a joint experimental and theoretical study on the [Ta2 B6 ](-) and [Ta2 B6 ] clusters. We found that the most stable structures of both the neutral and anion are D6h bipyramidal, similar to the recently discovered MB6 M structural motif in the Ti7 Rh4 Ir2 B8 solid compound.9.