Carbone stabilized B₂ and B₂²⁺ - isoelectronic analogues to diborabutyne and diborabutatriene

It has been reported that various unusual main group compounds can be stabilized by coordinating with ligands. Here, we report the use of carbone ligands in stabilizing diboron in its neutral and dicationic states by computational quantum mechanical calculations. The neutral [(L2C)B-2(CL2)] (L = CO, NHC, PMe3, and cAAC) has singlet non-planar cumulenic-type equilibrium geometry where CL2 groups are almost orthogonal to each other. MO analysis indicates that the [(L2C)B-2(CL2)] can be considered as formed by the interaction of the B-2 fragment in the (1)Sigma(+)(g) excited state with two CL2 ligands having sigma- and pi-type lone pairs. Accordingly, the pi delocalization in the C-B-B-C skeleton consists of two mutually orthogonal allylic anionic-type delocalizations along the C-B-B chain. Since one of the pi-delocalized MOs of allylic anionic C-B-B is majorly localized on the carbone carbon atom, the carbone ligands formally act as two-electron ligands. On the other hand, the ground state of [(L2C)B-2(CL2)](2+) shows a singlet planar/pseudo-planar cumulenic geometry when L = NHC and PMe3. The MO analysis indicates that the C-B-B-C skeleton is similar to that of butatriene, viz. one localized B-B pi MO, and two delocalized C-B-B-C pi MOs, indicating that each carbone acts as a four-electron ligand. Since CO and cAAC are good pi-acceptor ligands, [(L2C)B-2(CL2)](2+) ions (L = CO and cAAC) have triplet non-planar cumulenic ground states.