Geometric and electronic factors in the rational design of transition-metal-centered boron molecular wheels

The effects of the electronic and geometric factors on the global minimum structures of MB9(-) (M = V, Nb, Ta) are investigated using photoelectron spectroscopy and ab initio calculations. Photoelectron spectra are obtained for MB9(-) at two photon energies, and similar spectral features are observed for all three species. The structures for all clusters are established by global minima searches and confirmed by comparison of calculated and experimental vertical electron detachment energies. The VB9(-) cluster is shown to have a planar C2v V©B9(-) structure, whereas both NbB9(-) and TaB9(-) are shown to have Cs M©B9(-) type structures with the central metal atom slightly out of plane. Theoretical calculations suggest that the V atom fits perfectly inside the B9 ring forming a planar D(9h) V©B9(2-) structure, while the lower symmetry of V©B9(-) is due to the Jahn-Teller effect. The Nb and Ta atoms are too large to fit in the B9 ring, and they are squeezed out of the plane slightly even in the M©B9(2-) dianions. Thus, even though all three M©B9(2-) dianions fulfill the electronic design principle for the doubly aromatic molecular wheels, the geometric effect lowers the symmetry of the Nb and Ta clusters.