= Diborane
{title2=$\mathrm{B_2H_6}$}
<Diborane>, $\mathrm{B_2H_6}$, 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.
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