OsB₉ ^-: An Aromatic Osmium-Centered Monocyclic Boron Ring

Transition-metal-centered monocyclic boron wheels are important candidates in the family of planar hypercoordinate species that show intriguing structure, stability and bonding situation. Through the detailed potential energy surface explorations of MB₉ ^- (M = Fe, Ru, Os) clusters, we introduce herein OsB₉ ^- to be a new member in the transition-metal-centered borometallic molecular wheel gallery. Previously, FeB₉ ^- and RuB₉ ^- clusters were detected by photoelectron spectroscopy and the structures were reported to have singlet D ₉ symmetry. Our present results show that the global minimum for FeB₉ ^- has a molecular wheel-like structure in triplet spin state with C _s symmetry, whereas its heavier homologues are singlet molecular wheels with D ₉ symmetry. Chemical bonding analyses show that RuB₉ ^- and OsB₉ ^- display a similar type of electronic structure, where the dual σ + π aromaticity, originated from three delocalized σ bonds and three delocalized π bonds, accounts for highly stable borometallic molecular wheels.