Stabilization of Diborynes versus Destabilization of Diborenes by Coordination of Lewis Bases: Unravelling the Dichotomy

We have quantum chemically investigated the boron-boron bonds in B2, diborynes B2L2, and diborenes B2H2L2 (L=none, OH2, NH3) using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. B2 has effectively a single B-B bond provided by two half pi bonds, whereas B2H2 has effectively a double B=B bond provided by two half pi bonds and one sigma 2p-2p bond. This different electronic structure causes B2 and B2H2 to react differently to the addition of ligands. Thus, in B2L2, electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2H2L2, they lengthen and weaken the boron-boron bond. The aforementioned variations in boron-boron bond length and strength become more pronounced as the Lewis basicity of the ligands L increases. The different electronic structure of B2 and B2H2 makes them to react differently to the addition of ligands. Hence, in B2L2, electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2H2L2, they elongate and weaken the boron-boron bond.+image

Automated summary

The boron-boron bonds in B2, diborynes B2L2, and diborenes B2H2L2 (L=none, OH2, NH3) have been investigated quantum chemically. The study found that B2 has a single B-B bond, while B2H2 has a double B=B bond and a sigma 2p-2p bond. The different electronic structures of B2 and B2H2 cause them to react differently to ligand addition.