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Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B 9 O − ) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B 9 O − : the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B 9 O − is found surprisingly to be an umbrella-like structure ( C 6v , 1 A 1 ) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer ( C s , 1 A′) consisting of a BO unit bonded to a disk-like B 8 cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B 9 O − can be viewed as a central boron atom coordinated by a η 7 -B 7 ligand on one side and a BO ligand on the other side, [(η 7 -B 7 )-B-BO] − . The central B atom is found to share its valence electrons with the B 7 unit to fulfill double aromaticity, similar to that in half-sandwich [(η 7 -B 7 )-Zn-CO] − or [(η 7 -B 7 )-Fe(CO) 3 ] − transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron. 

Chirality plays an important role in nature. Nanoclusters can also exhibit chiral properties. We report herein a joint experimental and theoretical investigation on the geometric and electronic structures of B 31 − and B 32 − clusters, using photoelectron spectroscopy in combination with first-principles calculations. Two degenerate quasi-planar chiral C 1 enantiomers ( I and II , 1 A) with a central hexagonal vacancy are identified as the global minima of B 31 − . For B 32 − , two degenerate boat-like quasi-planar chiral C 2 structures ( VI and VII , 2 A) with a central hexagonal vacancy are also found as the global minima, with a low-lying chair-like C i B 32 − ( VIII , 2 A u ) also present in the experiment as a minor isomer. The chiral conversions in quasi-planar B 31 − and B 32 − clusters are investigated and relatively low barriers are found due to the high flexibility of these monolayer clusters, which feature multiple delocalized σ and π bonds over buckled molecular surfaces.