Electron deficiency

Electron deficiency is a term describing atoms or molecules having fewer than the number of electrons required for maximum stability. For each atom in a molecule, main group atoms having less than 8 electrons or transition metal atoms having less than 18 electrons are described as electron-deficient. For a whole molecule, molecules which have an incompletely filled set of bonding molecular orbitals are considered to be electron-deficient. Thus, CH₃ and BH₃ are electron-deficient, while methane (CH₄) and diborane (B₂H₆) are not. Not surprisingly, electron-deficient molecules are typically strongly electron-attracting (electrophilic).[1] As the most extreme form of electron deficiency one can consider the metallic bond.

Boranes and carboranes

Further information: Polyhedral skeletal electron pair theory

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.[2] 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.[3]

However it was shown starting in the 1940s that many such molecules have multicentre or delocalized bonds. The actual electronic structure of diborane has 2 (3-centre 2-electron) B-H-B bonds, as well as 4 (2-centre 2-electron) B-H bonds. There are thus a total of 6 bonding orbitals, which are precisely occupied by the 12 valence electrons, so that the molecule is actually electron-precise instead of electron-deficient.[4]

Another example is the extremely stable icosahedral B₁₂H₁₂^2- dianion, whose 26 cluster valence electrons exactly fill the 13 bonding molecular orbitals and is in no actual sense deficient in electrons; indeed it is thermodynamically far more stable than benzene. The same is true of its isoelectronic C₂B₁₀H₁₂ carborane analogues. More generally, nearly all carboranes, boranes, and other known and characterized polyboron clusters are similarly electron-precise. Some molecules that have no overall electron deficiency can nevertheless function as electron-acceptors at specific locations on the cluster, e.g., 1,2-C₂B₁₀H₁₂ (o-carborane), whose C-H bonds are slightly acidic owing to the local positive charge at the carbon vertices, which increases the polarity of these bonds. In contrast, the B-H groups in this molecule have a relatively high electron density and exhibit no electrophilic behavior.

Electron-withdrawing groups

Main article: Polar effect

The term electron-deficient has traditionally been used in organic 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.

References

1. R. N. Grimes (2016) Carboranes 3rd Edition, Elsevier, New York and Amsterdam, pp. 16-17.
2. 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.
3. Longuet-Higgins, H.C. (1957). "The structures of electron-deficient molecules". Quarterly Reviews, Chemical Society. 11 (2): 121–133. Retrieved 15 July 2020.
4. Lipscomb, William N. (11 December 1976). "The Boranes and their relatives (Nobel lecture)" (PDF). nobelprize.org. Nobel Foundation. pp. 224–245. Retrieved 16 July 2020. One of the simple consequences of these studies was that electron deficient molecules, defined as having more valence orbitals than electrons, are not really electron deficient.