Alkalide

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.^[1]

"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^[2] 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,^[3][4][5] 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".^[1]

Examples

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".^[6]

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.^[7]

References

1. ^ ^{a b c} M. Y. Redko, R. H. Huang, J. E. Jackson, J. F. Harrison, J. L. Dye (2003). "Barium azacryptand sodide, the first alkalide with an alkaline Earth cation, also contains a novel dimer, (Na₂)²⁻". J. Am. Chem. Soc. 125 (8): 2259–2263. doi:10.1021/ja027241m. PMID 12590555. 
2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
3. ^ J. L. Dye, J. M. Ceraso, Mei Lok Tak, B. L. Barnett, F. J. Tehan (1974). "Crystalline salt of the sodium anion (Na⁻)". J. Am. Chem. Soc. 96 (2): 608–609. doi:10.1021/ja00809a060. 
4. ^ F. J. Tehan, B. L. Barnett, J. L. Dye (1974). "Alkali anions. Preparation and crystal structure of a compound which contains the cryptated sodium cation and the sodium anion". J. Am. Chem. Soc. 96 (23): 7203–7208. doi:10.1021/ja00830a005. 
5. ^ J. L. Dye (1979). "Compounds of Alkali Metal Anions". Angew. Chem. Int. Ed. Engl. 18 (8): 587–598. doi:10.1002/anie.197905871. 
6. ^ M. Y. Redko, M. Vlassa, J. E. Jackson, A. W. Misiolek, R. H. Huang RH, J. L. Dye (2002). ""Inverse sodium hydride": a crystalline salt that contains H⁺ and Na⁻". J. Am. Chem. Soc. 124 (21): 5928–5929. doi:10.1021/ja025655. 
7. ^ J. Kim, A. S. Ichimura, R. H. Huang, M. Redko, R. C. Phillips, J. E. Jackson, J. L. Dye (1999). "Crystalline Salts of Na⁻ and K⁻ (Alkalides) that Are Stable at Room Temperature". J. Am. Chem. Soc. 121 (45): 10666–10667. doi:10.1021/ja992667v.