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

Abstract

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.