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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 compounds have also been synthesized containing a cation of the alkaline earth metal barium^[1].

[edit] "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.

[edit] 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]

[edit] 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 has allowed the isolation of kalide and natrides that are stable at room temperature.^[7]

[edit] References

^ ^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.
^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
^ 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.
^ 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.
^ J. L. Dye (1979). "Compounds of Alkali Metal Anions". Angew. Chem. Int. Ed. Engl. 18 (8): 587–598. doi:10.1002/anie.197905871.
^ 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.
^ 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.

[Redko-0] 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.

[1] Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

[2] 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.

[3] 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.

[4] J. L. Dye (1979). "Compounds of Alkali Metal Anions". Angew. Chem. Int. Ed. Engl. 18 (8): 587–598. doi:10.1002/anie.197905871.

[5] 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.

[6] 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.

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Contents

[edit] "Normal" chemistry: the case of Na+

[edit] Scope of alkalides

[edit] Examples

[edit] References

[edit] "Normal" chemistry: the case of Na⁺