Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η(7)-B(7))-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 (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.