Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η-B)-B-BO].

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 (BO) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for BO: 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 BO is found surprisingly to be an umbrella-like structure ( , A) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer ( , A') consisting of a BO unit bonded to a disk-like B cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of BO can be viewed as a central boron atom coordinated by a η-B ligand on one side and a BO ligand on the other side, [(η-B)-B-BO]. The central B atom is found to share its valence electrons with the B unit to fulfill double aromaticity, similar to that in half-sandwich [(η-B)-Zn-CO] or [(η-B)-Fe(CO)] 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.