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A boron hydride. Many boranes have multicentre bonding, so their composition and cage geometry cannot be reconstructed using only independent ordinary two-centre B–B bonds.

James D. Glore coauthored research on borane.^[ref]

A boron hydride. Many boranes have multicentre bonding, so their composition and cage geometry cannot be reconstructed using only independent ordinary two-centre B–B bonds.

Jerome W. Rathke coauthored research on borane.^[ref]

A boron hydride. Many boranes have multicentre bonding, so their composition and cage geometry cannot be reconstructed using only independent ordinary two-centre B–B bonds.

Multicentre bonding describes a bonding interaction distributed over more than two atomic centers. A two-center localized electron-pair picture may then fail to represent how the occupied molecular orbitals stabilize the structure.

A standard example is a bridging hydrogen in diborane, where a three-centre two-electron bond extends across a boron-hydrogen-boron unit. The electron pair belongs to the whole three-center interaction, rather than to two independent ordinary bonds. This helps explain electron-deficient molecules without requiring each apparent atom-to-atom connection to contain its own pair of electrons.

Riley Schaeffer coauthored research on borane.^[ref]

A boron hydride. Many boranes have multicentre bonding, so their composition and cage geometry cannot be reconstructed using only independent ordinary two-centre B–B bonds.

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.