Triple B≡B bond: from a perfect Lewis structure to a dominant π-back-donation. The need for a reference point

Coordination of a B₂ fragment by two σ-donor ligands could lead to complexes with a formal triple B≡B bond L→B≡B←L. Formation of L-B σ-bond leads to excess electrons around the B₂ central fragment. A subsequent direct π-charge transfer from B≡B moiety to the ligands L is a necessary condition for incorporation of BB fragment to the conjugated LBBL system. Quantum-chemical calculations (DFT, CCD, and CAS) show that the stabilization of a linear L-BB-L structure is possible but is accompanied by lowering of the B-B bond order. The ammonia-boryne structure H3N→BB←NH₃, which is studied as a model system, shows a perfect triple BB fragment relative to other LBBL adducts. The comparison of the conjugation between the B₂ fragment and two types of σ-donor ligands, with or without π-back-donation, provides an indication of the character of BB multiple bond. Three studied boryne molecules are calculated to have a high barrier for dissociation XXBBXX→XX + BBXX, yet a high reactivity of these compounds is indicated by the low-lying excited states of borynes (ΔE_S0-S1<1.5 eV). The largest gap ΔE_S0-S1∼2.6 eV is calculated for the room temperature stable bis-NHC-boryne.