On the way to the highest coordination number in the planar metal-centred aromatic Ta©B10− cluster: Evolution of the structures of TaBn− (n = 3–8)

The structures and chemical bonding of TaBn− (n = 3–8) clusters are investigated systematically to elucidate the formation of the planar metal-centred aromatic borometallic cluster, Ta©B10− (the © sign is used to designate the central position of the doped atom in monocyclic structures in M©Bn-type planar clusters), which was found previously to have the highest coordination number for a metal atom in a planar geometry. Photoelectron spectroscopy is combined with ab initio calculations to determine the global minima of the TaBn− clusters. We find that from TaB3− to TaB5− the boron atoms nucleate around the central Ta atom to form fan-like structures. A structural transition occurs at TaB6−, which is found to have a hexagonal structure, but with a boron atom in the centre and the Ta atom on the periphery. TaB7− is shown to have a three-dimensional boat-like structure, which can be viewed as a Ta atom coordinated to an elongated B7 cluster from above. The global minimum of the TaB8− cluster is found to be p...