The Transition-Metal-Like Behavior of B (NHC) in the Activation of CO: HOMO-LUMO Swap Without Photoinduction.

It is a current trend to explore multi-bonded and unsaturated main group compounds that can interact with small molecules, in order to find non-metal catalysts. Notably, Braunschweig et al. found that diboryne stabilized by N-heterocyclic carbenes (NHCs) can bind and activate CO. Here we explore the bonding nature of B (NHC) and its activation mechanism for CO from a novel theoretical perspective. While the ground state of B is of a single bond, the approach of NHCs excites B to its third excited state of a triple bond with two significant σ-holes at the two ends. The subsequent electrostatic attraction drives the formation of B (NHC) . However, only one of the two π bonds (HOMOs) of B (NHC) fits to one of the degenerate LUMOs of CO. Interestingly, the strong steric repulsion between CO and B (NHC) leads to the HOMO-LUMO swap in the latter. Subsequently, both HOMO and HOMO-1 of B (NHC) can effectively interact with the two π* anti-bonding orbitals (LUMO and LUMO+1) of CO, resulting in substantial electron back-donation and the ultimate activation of CO.