“Electron deficiency”的意思、由来-开放百科全书

Electron deficiency is a term describing atoms or molecules having fewer than the number of electrons required for maximum stability. At the atomic level, main group atoms having less than 8 electrons or transition metal atoms having less than 18 electrons are described as electron-deficient. At the molecular level, 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

For many years, "electron-deficiency" was often used as a general descriptor for boron hydrides and other molecules featuring multicenter bonding (in which a pair of bonding electrons extends over more than two atoms, as in three-center two-electron bonds), as a way of distinguishing such molecules from conventionally bonded compounds such as hydrocarbons.^[2] However, this usage was incorrect, as many (indeed most) of the molecules formerly labeled "electron-deficient", such as boranes, are actually electron-precise. An 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

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. ^H. C. Longuet-Higgins (1957) "The structures of electron-deficient molecules", Quarterly Reviews, Chemical Society, Volume 11, issue 2, pp. 121–133.