Borane

Article By:

Kennedy, John D. School of Chemistry, University of Leeds, Leeds, United Kingdom.

Last reviewed:October 2019

DOI:https://doi.org/10.1036/1097-8542.090900

The general name given to any one of the binary boron hydrides of the general formula B_nH_m. Boron is the only element other than carbon that exhibits an extensive series of molecular hydrides. In contrast to the chains and rings of carbon atoms that typify carbon hydride chemistry, the structures of the boron hydrides are based on polyhedral clusters of boron atoms. These borane clusters are characterized by triangular faces, and such polyhedra are known as deltahedra. The boranes typically have a surface of exo B-H bonds directed radically outward from the deltahedral cluster. Deltahedra are closed if they consist entirely of triangular faces, whereas open deltahedra can also typically incorporate four-, five-, and six-membered open faces. Associated with such open faces are additional BHB bridging and/or endo B-H hydrogen atoms, which are held in positions approximately tangential to the cluster surface. These deltahedral cluster structures arise because assemblies of boron and hydrogen atoms have fewer electrons than bonding atomic orbitals available to hold them together. For this reason, they are sometimes described as electron-deficient. Carbon atoms have the same number of valence electrons as atomic orbitals available for bonding and can therefore form the more familiar electron-precise chains and rings of the hydrocarbons, or more electron-rich structures with active lone pairs if atoms of elements to the right of carbon in the periodic table, such as oxygen and nitrogen, are also incorporated. See also: Boron; Organic chemistry; Structural chemistry

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