Do multiple bonds to the boron atom exist?

There are occasional publications in the scientific literature stating that newchemical compounds containing multiple bonds to a boron atom have been synthesized or confirmed using quantum-chemical methods. In these, the concept of molecular orbital (MO) in conjunction with bond length considerations is used. Another view of the electronic structure of molecules comes from the Quantum Chemical Topology world, where the chemical bond is treated in real space using such concepts as the basin, critical point, and attractor.

In this work the topological analysis of ELF has been used to study the electronic structure of 124 molecules containing BA (A= B, C, N, O, F(Cl)) bonds. The data was obtained at the DFT computational level.The formally triple (B≡A), double (B=A), single (B-A) and one electron (B.A) bonds have been considered. For each bond the bonding basin of the ELF field has been localised. The values obtained for the basin populations of the BA bonds enabled verification of the formal bond orders without evoking the concept of the molecular orbital.

Surprisingly, multiple bonds have NOT been found for the boron-oxygen, BO, and boron-fluorine, BF, and boron-chlorine BCl bonds, but are supported for the boron-boron, BB, boron-carbon, BC, and boron-nitrogen BN, bonds. Our analysis shows that moving from the B to the F (Cl) atoms the population of the BA bond decreases (Figure 1). The chemical bonding is associated with a smaller amount of paired electrons, and therefore exhibit smaller topological bond orders. The covalency of the bond to the B atom, measured by the population of the bonding basin, decreases from the B to the F atom. A significant change between the bonding types occurs between the BN and BO bonds (Figure 1).

The results present the different view of the nature of chemical bonds formed by the boron atom. In fact, our results contradict the present paradigm of the bonding theory based on the molecular orbital concept for the BO and BF(Cl) bonds. The conclusions from our studies provide an incentive for further investigations on the nature of chemical bonding in general.