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 π bonds, whereas B₂H₂ has effectively a double B=B bond provided by two half π bonds and one σ 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.