electrondeficient

Electron-deficient describes a chemical species that has fewer electrons around one or more atoms than would be required for a stable valence shell, typically lacking a complete octet. This deficiency creates empty or low-energy orbitals and often leads to increased electrophilicity. Electron-deficient compounds are usually strong Lewis acids, meaning they readily accept electron density from Lewis bases.

Common examples include boron trihalides such as boron trifluoride (BF3) and boron trichloride (BCl3), as well as boranes like borane (BH3) and diborane (B2H6). In these species, the central boron atom has only six valence electrons in its octet, leaving an empty orbital that can accept electron pairs. Carbocations, such as the methyl cation (CH3+), are another class of electron-deficient species, possessing only six electrons around carbon and seeking electron density from nucleophiles.

Electron deficiency is also important in certain metal-containing fragments and in compounds that form three-center two-electron bonds (3c-2e), which help stabilize electron-poor centers. These species often undergo reactions with nucleophiles, add to unsaturated systems, or form adducts with donors such as phosphines, amines, or ethers. Stabilization can occur through back-donation from donor ligands or by forming more complete bonding networks via adduct formation.

In practice, the term is used to describe both transient reactive intermediates and stable Lewis acids. The degree of deficiency can be influenced by substituents, coordination environment, and surrounding ligands, which modify the species’ reactivity and acidity.