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 and diborane are not. Not surprisingly, electron-deficient molecules are typically strongly electron-attracting. 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 bonds joining all atoms. For example, diborane would require a minimum of 7 localized bonds with 14 electrons to join all 8 atoms, but there are only 12 valence electrons.
However it was shown starting in the 1940's that many such molecules have multicentre or delocalized bonds. The actual electronic structure of diborane has 2 B-H-B bonds, as well as 4 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.
Another example is the extremely stable icosahedral B12H122- 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, 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.