Diborane (${\mathrm{B}}_2{\mathrm{H}}_6$)

Diborane, ${\mathrm{B}}_2{\mathrm{H}}_6$, is an electron-deficient borane with four terminal B–H connections and two bridging hydrogen atoms. The bridges are described by three-centre two-electron bonds, with one bonding electron pair shared over each B–H–B unit.

The valence-electron count explains why the multicentre model is useful: $2(3)+6(1)=12$ valence electrons. Four ordinary terminal bonds use eight, leaving four for the two bridges. Assigning a separate ordinary two-electron bond to every boron–hydrogen contact would demand more electrons than the neutral molecule has. Diborane therefore provides a concrete counterexample to treating every drawn connection as an independent localized electron-pair bond.