B2Hn

B2Hn is a class of boron hydride compounds that contain two boron atoms and a variable number of hydrogen atoms. The general formula reflects the fact that the hydrogen stoichiometry can change depending on the conditions of synthesis, the presence of ligands, or the reaction pathway. The most well‑known member of this family is diborane (B2H6), which is the simplest polyhydride of boron. Other derivatives can include lower hydrogen content, such as B2H5 or higher with additional bridging hydrogens in larger clusters.

The molecules are typically synthesized by reducing boron halides with alkali metals or by reacting boron trichloride with a hydride source such as lithium aluminium hydride in a nonpolar solvent. The reaction environment, temperature, and choice of reducing agent influence the hydrogen content and the resulting structure.

Structural features of B2Hn compounds are characterized by bridging hydrogen atoms that form multi‑center bonds between the two boron atoms. In B2H6, for instance, two terminal hydrogens bound to each boron are complemented by two bridging hydrogens that create three‑center, two‑electron bonds, a motif common in low‑coordination boranes. As the hydrogen content varies, the geometry and electronic distribution adapt accordingly.

These boron hydrides are of interest for several reasons. They serve as model systems for studying unusual bonding situations such as multicenter bonding and electron‑deficient networks. In addition, some low‑hydrogen species are considered as potential hydrogen storage materials because of their high hydrogen density. Their reactivity also makes them useful reagents in organic synthesis, where they can act as hydride donors or as sources of boron for catalysis.

Safety considerations note that B2Hn compounds can be pyrophoric and must be handled under inert atmosphere. Proper ventilation and protective equipment are required to mitigate the risks of exposure to flammable or toxic gases. Researchers continue to investigate the properties, synthesis routes, and applications of these intriguing boron hydrides.