Electron deficiency

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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[]

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[]

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[]