Electron deficiency

Electron deficiency (and electron-deficient) is jargon that is used in two contexts: species that violate the octet rule because they have too few valence electrons and species that happen to follow the octet rule but have electron-acceptor properties, forming donor-acceptor charge-transfer salts.

Octet rule violations
Triphenylborane is classified as electron deficient.
Main article: Boranes

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.[1] 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.[2] A similar situation exists in trimethylaluminium. The electron deficiency in such compounds is similar to metallic bonding.

Electron-acceptor molecules
Structure of the charge-transfer complex between pyrene with the electron-deficient 1,3,5-trinitrobenzene.[3]

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.[4] 1,3,5-Trinitrobenzene and related polynitrated aromatic compounds are often described as electron-deficient.[5] 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."[6]

References
  1. Housecroft, Catherine E.; Sharpe, Alan G. (2005). Inorganic Chemistry (2nd ed.). Pearson Prentice-Hall. p. 326. ISBN 0130-39913-2. An electron-deficient species possesses fewer valence electrons than are required for a localized bonding scheme.
  2. Longuet-Higgins, H.C. (1957). "The structures of electron-deficient molecules". Quarterly Reviews, Chemical Society. 11 (2): 121–133. doi:10.1039/qr9571100121. Retrieved 15 July 2020.
  3. Rather, Sumair A.; Saraswatula, Viswanadha G.; Sharada, Durgam; Saha, Binoy K. (2019). "Influence of molecular width on the thermal expansion in solids". New Journal of Chemistry. 43 (44): 17146–17150. doi:10.1039/C9NJ04888J. S2CID 208752583.
  4. Stalder, Romain; Mei, Jianguo; Graham, Kenneth R.; Estrada, Leandro A.; Reynolds, John R. (2014). "Isoindigo, a Versatile Electron-Deficient Unit for High-Performance Organic Electronics". Chemistry of Materials. 26: 664–678. doi:10.1021/cm402219v.
  5. Goetz, Katelyn P.; Vermeulen, Derek; Payne, Margaret E.; Kloc, Christian; McNeil, Laurie E.; Jurchescu, Oana D. (2014). "Charge-Transfer Complexes: New Perspectives on an Old Class of Compounds". J. Mater. Chem. C. 2 (17): 3065–3076. doi:10.1039/C3TC32062F.
  6. Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 412, ISBN 978-0-471-72091-1
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