Inorganic chemistry, aradical anion is afree radical species^[1] that carries anegative charge. Radicalanions are encountered inorganic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g.sodium naphthenide. An example of a non-carbon radical anion is thesuperoxide anion, formed by transfer of one electron to anoxygen molecule. Radical anions are typically indicated by${\displaystyle M^{\bullet -}}$.

Manyaromatic compounds can undergoone-electron reduction byalkali metals. The electron is transferred from the alkali metal ion to an unoccupied antibonding p-p п* orbital of the aromatic molecule. This transfer is usually only energetically favorable if the aprotic solvent efficiently solvates the alkali metal ion. Effective solvents are those that bind to the alkali metal cation:diethyl ether <THF <1,2-dimethoxyethane <HMPA. In principle any unsaturated molecule can form a radical anion, but the antibonding orbitals are only energetically accessible in more extensive conjugated systems. Ease of formation is in the orderbenzene <naphthalene <anthracene <pyrene, etc. Salts of the radical anions are often not isolated as solids but used in situ. They are usually deeply colored.

• Naphthalene in the form of
  • Lithium naphthalene is obtained from the reaction of naphthalene withlithium.
  • Sodium naphthalene is obtained from the reaction ofnaphthalene withsodium.
  • Sodium1-methylnaphthalene and 1-methylnaphthalene are more soluble than sodium naphthalene and naphthalene, respectively.^[2]
• biphenyl as its lithium salt.^[3]
• acenaphthylene is a milder reductant than the naphthalene anion.
• anthracene in the form of its alkali metal salts.^[4]
• pyrene as its sodium salt.^[5]
• Perylene in the form of its alkali metal (M = Li, Na, Cs) etherates.^[6]

Cyclooctatetraene is reduced by elementalpotassium to the dianion. The resulting dianion is a 10-pi electron system, which conforms to theHuckel rule foraromaticity.Quinone is reduced to asemiquinone radical anion.Semidiones are derived from the reduction of dicarbonyl compounds.

The pi-radical anions are used as reducing agents in specialized syntheses. Being soluble in at least some solvents, these salts act faster than the alkali metals themselves. The disadvantages are that the polycyclic hydrocarbon must be removed. The reduction potential of alkali metal naphthalene salts is about 3.1 V (vs Fc^+/0). The reduction potentials of the larger systems are lower, for example acenaphthalene is 2.45 V.^[7] Many radical anions are susceptible to further reduction to dianions.

Addition of a proton source (even water) to a radical anion results in protonation, i.e. the sequence of reduction followed by protonation is equivalent tohydrogenation. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene. Radical anions and their protonation are central to theBirch reduction.

Radical anions of polycyclic aromatic compounds function as ligands inorganometallic chemistry.^[8]

• Reactive intermediates
• Mass spectrometry

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