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

• Published in: The Journal of chemical physics 2013-09, Vol.139 (10)
• Main Authors: Li, Wei-Li, Ivanov, Alexander S., Federič, Jozef, Romanescu, Constantin, Černušák, Ivan, Boldyrev, Alexander I., Wang, Lai-Sheng
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4820401

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 pyramidal with the Ta atom interacting with a B8 monocyclic ring. Starting from this structure, additional boron atoms simply enlarge the boron ring to form the slightly pyramidal TaB9− cluster and eventually the perfectly planar Ta-centred B10-ring aromatic cluster, Ta©B10−. It is shown that boron atoms do not nucleate smoothly around a Ta atom on the way to the decacoordinated Ta©B10− molecular wheel, but rather the competition between B–B interactions and Ta–B interactions determines the most stable structures of the smaller TaBn− (n = 3–8) clusters.