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Alkalides are chemical compounds in which alkali metals are anions (that is, they bear a negative charge). Such species are notable because alkali metals were previously thought to appear in salts only as cations. Alkalide-like compounds have also been synthesized containing an anion of the alkaline earth metal barium^[1].

Alkali metals are well known to form salts. Table salt, or sodium chloride Na⁺Cl^-, illustrates the usual role of an alkali metal such as sodium: its positive charge is balanced by a negatively charged ion in the empirical formula for this ionic compound. The traditional explanation for this phenomenon is that the loss of one electron from elemental sodium to produce a cation with a single positive charge produces a stable closed-shell electron configuration. Sodium was thought to always form singly charged cations until the discovery of alkalides^[2] and the same arguments apply to the remainder of the alkali metals.

Known alkalides include Na^-, K^-, Rb^-, and Cs^-. These species are called natride, kalide, rubidide, and caeside, respectively. “Lithides” are not currently known. The known alkalides, first discovered in the 1980s, are of theoretical interest due to their unusual stoichiometry and low ionization potentials. Alkalides species are chemically related to the electrides.

A typical alkalide is the sodium natride salt [Na(2,2,2-crypt)]⁺Na^-. This salt contains both Na⁺ and Na^-. The cryptand isolates and stabilizes the Na⁺, preventing its reduction by the Na^-. Dimers of cationic and anionic sodium have also been observed^[1], as has an H⁺Na^- salt known as "inverse sodium hydride"^[3].

Normally, alkalides are thermally labile due to the high reactivity of the alkalide anion, which is theoretically able to break most covalent bonds including the C-O bonds in a typical cryptand. The introduction of a special cryptand ligand has allowed the isolation of kalide and natrides that are stable at room temperature.^[4]

1. ^ ^{a b} Redko MY, Huang RH, Jackson JE, Harrison JF, Dye JL. (2003). Barium azacryptand sodide, the first alkalide with an alkaline Earth cation, also contains a novel dimer, (Na(2))(2-). J Am Chem Soc 125(8):2259-63.
2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
3. ^ Redko MY, Vlassa M, Jackson JE, Misiolek AW, Huang RH, Dye JL.(2002). "Inverse sodium hydride": a crystalline salt that contains H(+) and Na(-). J Am Chem Soc 124(21):5928-9.
4. ^ Kim, J.; Ichimura, A. S.; Huang, R. H.; Redko, M.; Phillips, R. C.; Jackson, J. E. and Dye, J. L. (1999). Crystalline Salts of Na^- and K^- (Alkalides) that Are Stable at Room Temperature. J Am Chem Soc 121:10666-10667.