An alkalide is a chemical compound in which alkali metal atoms are (negative ions) with a electric charge or oxidation state of −1. Until the first discovery of alkalides in the 1970s,[Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. .] These types of compounds are of theoretical interest due to their unusual stoichiometry and low ionization potentials. Alkalide compounds are chemically related to the , salts in which trapped electrons are effectively the anions.
"Normal" alkali metal compounds
Alkali metals form many well-known stable salts.
Sodium chloride (common table salt), , illustrates the usual role of an alkali metal such as sodium. In the empirical formula for this
ionic compound, the positively charged
sodium ion is balanced by a negatively charged
chloride ion. The traditional explanation for stable is that the loss of one
electron from elemental sodium to produce a cation with charge of +1 produces a stable
valence shell electron configuration.
Nomenclature and known cases
There are known alkalides for some of the alkali metals:
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Sodide or natride,
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Potasside or kalide,
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Rubidide,
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Caeside,
Alkalides of the other alkali metals have not yet been discovered:
Examples
Normally, alkalides are thermally labile due to the high reactivity of the alkalide anion, which is theoretically able to break most
including the carbon–oxygen bonds in a typical
cryptand. The introduction of a special cryptand ligand containing amines instead of ether linkages has allowed the isolation of kalides and natrides that are stable at room temperature.
Several alkalides have been synthesized:
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A compound in which hydrogen ions are encapsulated by adamanzane, known as hydrogen natride or "inverse sodium hydride" (hydrogen sodide or hydrogen natride ), has been observed.
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Sodium-crypt natride, Na(cryptand[2.2.2)]+Na−, has been observed. This salt contains both and . The cryptand isolates and stabilizes the , preventing it from being reduced by the .
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Barium azacryptand-sodide, Ba2+H5Azacryptand[2.2.2]−Na−⋅2CH3NH2, has been synthesized.
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Dimers of cationic and anionic sodium have been observed.
