Electron deficiency

Electron deficiency is a term describing molecules having fewer than the number of electrons required for maximum stability. In quantum mechanical terms, electron-deficient molecules have an incompletely filled set of bonding molecular orbitals. Thus, CH₄⁺ and BH₃ are electron-deficient, while B₂H₆ (a dimer of BH₃) is not. Electron-deficient molecules are typically strongly electron-attracting (electrophilic). For many years," electron-deficiency" was a label used to describe boron hydrides and other molecules featuring multicenter bonding, in which a pair of electrons binds more than two atoms together, as in 3-center-2-electron bonds; however, many (indeed most) such molecules are actually electron-precise and not electron-deficient. An example is the extremely stable B₁₂H₁₂^2- dianion, which has icosahedral symmetry and is highly nonreactive toward electron donors. More generally, nearly all carboranes, boranes, and other known and characterized boron clusters are electron-precise.[

The term electron-deficient is used inorganic chemistry to indicate a pi-system such as an alkene or arene that has electron-withdrawing groups attached, as found in nitrobenzene or acrylonitrile. Instead of showing the nucleated character common with simple C=C bonds, electron-deficient pi-systems may be electrophilic and susceptible to nucleophilic attack, as is seen in the Michael addition or in nucleophilic aromatic substitution.

As the most extreme form of electron deficiency one can consider the metallic bond.

Literature

[1] R. N. Grimes (2016) Carboranes 3rd Edition, Elsevier, New York and Amsterdam, pp. 16-67.

• H. C. Longuet-Higgins (1957). "The structures of electron-deficient molecules". Quarterly Reviews, Chemical Society. 11 (2): 121–133. doi:10.1039/QR9571100121.