Electron deficiency

Octet rule violations

thumb|left|[[Triphenylborane]] is classified as electron deficient.|144px Main article: Octet rule#Exceptions

Traditionally, "electron-deficiency" is used as a general descriptor for boron hydrides and other molecules which do not have enough valence electrons to form localized (2-centre 2-electron) bonds joining all atoms. For example, diborane (B2H6) would require a minimum of 7 localized bonds with 14 electrons to join all 8 atoms, but there are only 12 valence electrons. A similar situation exists in trimethylaluminium. The electron deficiency in such compounds is similar to metallic bonding.

Electron-acceptor molecules

Alternatively, electron-deficiency describes molecules or ions that function as electron acceptors. Such electron-deficient species obey the octet rule, but they have (usually mild) oxidizing properties. 1,3,5-Trinitrobenzene and related polynitrated aromatic compounds are often described as electron-deficient. Electron deficiency can be measured by linear free-energy relationships: "a strongly negative ρ value indicates a large electron demand at the reaction center, from which it may be concluded that a highly electron-deficient center, perhaps an incipient carbocation, is involved."

References

References

1. (2005). "Inorganic Chemistry". Pearson Prentice-Hall.
2. (1957). "The structures of electron-deficient molecules". Quarterly Reviews, Chemical Society.
3. (2019). "Influence of molecular width on the thermal expansion in solids". New Journal of Chemistry.
4. (2014). "Isoindigo, a Versatile Electron-Deficient Unit for High-Performance Organic Electronics". Chemistry of Materials.
5. (2014). "Charge-Transfer Complexes: New Perspectives on an Old Class of Compounds". J. Mater. Chem. C.
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