On the nature of B-Ccarbene bonding in a stable neutral diborene.

We report the bonding interactions within [R(H)B=B(H)] and [R] (R=:C(NHCH)(2)) as a ligand in a newly synthesized stable neutral diborene. By using theoretical analyses, we have found the nature of the B-C(carbene) bonding, and, more importantly, the key to realize multiple bonds for chemical elements. With character of almost equal covalency and ionicity, the stabilizing orbital interaction term, DeltaE(orb), of B-C(carbene), is mainly given by sigma-symmetry orbital interactions; the donor-acceptor interaction is weak and contributes small to DeltaE(orb). In the weak donor-acceptor interaction, the B-->C(carbene) pi backdonation is stronger than the B<--C(carbene) sigma donation. Thus, in effect, the bond emerges in the B(delta+)-C(carbene)(delta-) dipole. Inspection of the correlation lines of the orbital correlation diagram for the B-C(carbene) bonding indicates that the strength of the bonding orbitals in the central BB unit is weakened due to the coordination of the carbenes, and the center is unstabilized by the carbene ligand. This is contrary to the conventional view on the mechanism of coordination and the Dewar-Chatt-Duncanson model. However this unstabilizing effect should be responsible for the stability of the B=B double bond in the stable neutral diborene. This is because the very short bond lengths arising from multiple bonds will lead to a very strong Pauli repulsion, and, ultimately, destruction of chemical bonds. It can therefore be concluded that, actually, to prevent the very short bond lengths is the true reason for the successful realization of multiple bonds for main-group elements such as boron.