Diboryne

Diborynes are low-valent main-group compounds that contain a formal boron–boron triple bond (B≡B). In most isolable examples, the B≡B unit is stabilized by strong σ-donor ligands and is often described as L→B≡B←L; N-heterocyclic carbenes (NHCs) are among the most common supporting ligands. Structurally, many diborynes exhibit an approximately linear B–B ligand arrangement, which makes them a useful point of comparison to other main-group multiple-bond systems where π-bonding can be harder to sustain.

Their bonding description has been examined extensively. While the B≡B linkage is commonly discussed in terms of a σ bond and two π interactions, experimental and theoretical work has emphasized that ligand donation plays an important role in stabilizing this framework, and that the B–B multiple bond is generally weaker than the C≡C bond in alkynes. At the same time, diborynes can retain low-lying acceptor character at boron, which helps explain their tendency to engage in donor–acceptor interactions and their broader reactivity patterns.

Reactivity is often centered at the B≡B unit and includes multi-electron addition and insertion processes (for example, chalcogen insertion), as well as small-molecule transformations that are uncommon for typical p-block compounds, such as metal-free CO binding and coupling in selected systems. Diborynes and related π-complexes can also show distinctive photophysical behavior, and some reported complexes display strong room-temperature phosphorescence. Since the first isolable, fully characterized diboryne was reported in 2012, a range of synthetic approaches and reactivity motifs have been developed, allowing these compounds to be discussed in terms of synthesis, bonding, reactivity, and photophysical properties.