Mimicking the C 2 molecule: M 2 B 2 and M 3 B 2 + clusters (M = Li, Na) and the reactivity of the N-heterocyclic carbene bound Li 2 B 2 complex.

C 2 has attracted considerable attention from the scientific community for its debatable bonding situation. Herein, we show that the global minima of M 2 B 2 and M 3 B 2 + (M = Li, Na) possess similar covalent bonding patterns to C 2 . Because of strong charge transfer from M 2 /M 3 to B 2 dimer, they can be better described as [M 2 ] 2+ [B 2 ] 2- and [M 3 ] 3+ [B 2 ] 2- salt complexes with the B 2 2- core surrounded perpendicularly by two and three M + atoms, respectively. The energy decomposition analyses in combination with the natural orbital for chemical valence theory give four bonding components in C 2 , M 2 B 2 , and M 3 B 2 + clusters. However, the fourth component does not arise from a bonding interaction but from polarization/hybridization. Considering the effect of Pauli repulsion in σ-space, the attractive covalent interaction in these molecules mainly comes from the two π-bonds. We further presented stable N-heterocyclic carbene (NHC) and triphenylphosphine (PPh 3 ) ligands bound Li 2 B 2 (NHC) 2 and Li 2 B 2 (PPh 3 ) 2 complexes. A comparative study of reactivity towards L = CO 2 , CO, and N 2 between Li 2 B 2 (NHC) 2 and B 2 (NHC) 2 is also performed. L-Li 2 B 2 (NHC) 2 is highly stable against L dissociation at room temperature for L = CO 2 and CO, and the stability is markedly higher than that in L-B 2 (NHC) 2 . The larger B 2 →L π-backdonation in L-Li 2 B 2 (NHC) 2 also makes L more activated than in L-B 2 (NHC) 2 .