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

A neutral boron carbonyl complex B 4 (CO) 3 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 2v structure with three CO ligands terminally coordinated to a rhombus B 4 core. It has a closed-shell singlet ground state that correlates to an excited state of B 4 . Bonding analyses on B 4 (CO) 3 as well as the previously reported B 4 and B 4 (CO) 2 indicate that the electronic structure of rhombus tetraboron cluster changes from a close-shell singlet to an open-shell singlet in B 4 (CO) 2 and to a close-shell singlet in B 4 (CO) 3 , demonstrating that the electronic structures of boron clusters can be effectively tuned via sequential CO ligand coordination.