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 B2(NHC)2 and its activation mechanism for CO from a novel theoretical perspective. While the ground state of B2 is of a single bond, the approach of NHCs excites B2 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 B2(NHC)2. However, only one of the two π bonds (HOMOs) of B2(NHC)2 fits to one of the degenerate LUMOs of CO. Interestingly, the strong steric repulsion between CO and B2(NHC)2 leads to the HOMO–LUMO swap in the latter. Subsequently, both HOMO and HOMO−1 of B2(NHC)2 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.