An alkalide is a chemical compound 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 compounds have also been synthesized containing a cation of the alkaline earth metal barium.
"Normal" chemistry: the case of Na+
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 and the same arguments apply to the remainder of the alkali metals.
Scope of alkalides
Known alkalides include Na−, K−, Rb−, and Cs−. These species are called sodide or natride, potasside or kalide, rubidide, and caeside, respectively. “Lithides”, compounds containing Li−, are not currently known. The known alkalides, first discovered in the 1970s, are of theoretical interest due to their unusual stoichiometry and low ionization potentials. Alkalide species are chemically related to the electrides, salts containing trapped electrons as the "anions".
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, as has an H+Na− salt known as "inverse sodium hydride".
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 containing amines instead of ether linkages has allowed the isolation of kalide and natrides that are stable at room temperature.
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