Borane

A borane is an inorganic chemical compound of boron and hydrogen. The lighter boranes are notably unstable - diborane ignites in air to burn with a green flame - but higher ones are much less so. Decaborane is stable and crystalline, reacting with neither air nor water.

Chemistry

The chemistry of boranes is dominated by boron possessing only three valence electrons, but four valence orbitals. Imagining a covalently-bonded system for BH3, boron, with its three valence electrons, will bond to three hydrogen atoms, which in turn each share one electron with the boron to give it a valence electron count of six. However, the octet rule states that a top row atom such as boron must fill its valence orbitals for maximum stability, and therefore possess eight valence electrons, yet the boron in discrete BH3 has only six valence electrons, and hence a vacant p-orbital. The unsaturation of borane results in a highly-reactive species that only exists in the gas phase. It readily dimerises to form diborane and, with larger numbers of boron atoms, clusters.

Related Topics:
Octet rule - Unsaturation - Diborane - Clusters

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Cluster formation overcomes the electron deficiency of boranes by utilising a molecular orbital bonding scheme that gives rise to 3-center-2-electron bonds. Using empirical rules developed by K. Wade and later improved by M. Mingos, known as Polyhedral skeletal electron pair theory or Wade's/Mingos' rules, the structure of a boron cluster can often be unambiguously determined from the chemical formula.

Related Topics:
3-center-2-electron bond - Polyhedral skeletal electron pair theory

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