δ Aromaticity Spied In Metal Clusters

Computational studies and gas-phase photoelectron spectroscopy experiments have confirmed the existence of delocalized bonding involving the d atomic orbitals of transition metals. Research on the planar Ta₃O₃^- cluster (shown at left) by Alexander I. Boldyrev of Utah State University, Lai-Sheng Wang of Washington State University, Richland, and coworkers provides the first experimental evidence of a δ-aromatic molecule and suggests that δ aromaticity exists in other planar transition-metal complexes (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200700442). The concept of aromaticity was introduced into organic chemistry to describe delocalized bonding in planar, cyclic, conjugated molecules such as benzene. Aromaticity has since been used to describe bonding in main-group and organometallic compounds, in particular in all-metal clusters. These compounds may exhibit multiple σ and π aromaticity or antiaromaticity involving s, p, or d atomic orbitals. In the case of Ta3O3–, the researchers show that the d orbitals participate in π and δ delocalized bonding (δ molecular orbital at right). The next challenge, the researchers say, is to look for Φ aromaticity, which might occur in metals with f orbitals.