Ligand-Induced Tuning of the Electronic Structure of Rhombus Tetraboron Cluster.

A neutral boron carbonyl complex B (CO) is generated in the gas phase and is characterized by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy and quantum chemical calculations. The complex is identified to have a planar C structure with three CO ligands terminally coordinated to a rhombus B core. It has a closed-shell singlet ground state that correlates to an excited state of B . Bonding analyses on B (CO) as well as the previously reported B and B (CO) indicate that the electronic structure of rhombus tetraboron cluster changes from a close-shell singlet to an open-shell singlet in B (CO) and to a close-shell singlet in B (CO) , demonstrating that the electronic structures of boron clusters can be effectively tuned via sequential CO ligand coordination.