A three-centre two-electron bond has two bonding electrons shared across three atomic centers. Combining three suitable atomic orbitals gives bonding, nonbonding, and antibonding combinations; occupation of the lowest combination by one electron pair can stabilize the three-center unit.
The bridging boron-hydrogen-boron interactions in diborane are the classic example. Counting each bridge as two ordinary two-center electron-pair bonds would require more electrons than are available. Multicentre bonding resolves the count while preserving a quantum-mechanical description in terms of orbitals extending across the atoms.
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