electron deficiency
{{Short description|Species that violate the octet rule or have electron-acceptor properties}}
In chemistry, electron deficiency (and electron-deficient) is jargon that is used in two contexts: chemical 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
file:Structural formula of triphenylborane.svg is classified as electron deficient.|144px]]
{{main|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.{{cite book |last1=Housecroft |first1=Catherine E. |last2=Sharpe |first2=Alan G. |title=Inorganic Chemistry |date=2005 |publisher=Pearson Prentice-Hall |isbn=0130-39913-2 |page=326 |edition=2nd |quote=An electron-deficient species possesses fewer valence electrons than are required for a localized bonding scheme.}} 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.{{cite journal |last1=Longuet-Higgins |first1=H.C. |title=The structures of electron-deficient molecules |journal=Quarterly Reviews, Chemical Society |date=1957 |volume=11 |issue=2 |pages=121–133 |doi=10.1039/qr9571100121 |url=https://pubs.rsc.org/en/content/articlelanding/1957/qr/qr9571100121 |access-date=15 July 2020}} A similar situation exists in trimethylaluminium. The electron deficiency in such compounds is similar to metallic bonding.{{cn|date=April 2025}}
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.{{cite journal |doi=10.1021/cm402219v|title=Isoindigo, a Versatile Electron-Deficient Unit for High-Performance Organic Electronics |year=2014 |last1=Stalder |first1=Romain |last2=Mei |first2=Jianguo |last3=Graham |first3=Kenneth R. |last4=Estrada |first4=Leandro A. |last5=Reynolds |first5=John R. |journal=Chemistry of Materials |volume=26 |pages=664–678 }} 1,3,5-Trinitrobenzene and related polynitrated aromatic compounds are often described as electron-deficient.{{cite journal |doi=10.1039/C3TC32062F|title=Charge-Transfer Complexes: New Perspectives on an Old Class of Compounds|year=2014|last1=Goetz|first1=Katelyn P.|last2=Vermeulen|first2=Derek|last3=Payne|first3=Margaret E.|last4=Kloc|first4=Christian|last5=McNeil|first5=Laurie E.|author5-link=Laurie McNeil|last6=Jurchescu|first6=Oana D.|author6-link=Oana Jurchescu|journal=J. Mater. Chem. C|volume=2|issue=17|pages=3065–3076}} 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."{{March6th|page=412}}