Electron deficiency

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.

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.