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Cavities and channels in an electride

An electride is an ionic compound in which an electron is the anion.[1] Solutions of alkali metals in ammonia are electride salts.[2] In the case of sodium, these blue solutions consist of [Na(NH3)6]+ and solvated electrons:

Na + 6 NH3 → [Na(NH3)6]+,e

The cation [Na(NH3)6]+ is an octahedral coordination complex.

Solid salts[edit]

Addition of a complexant like crown ether or 2,2,2-cryptand to a solution of [Na(NH3)6]+e affords [Na(crown ether)]+e or [Na(2,2,2-crypt)]+e. Evaporation of these solutions yields a blue-black paramagnetic salt with the formula [Na(2,2,2-crypt)]+e.

Most solid electride salts decompose above 240 K, although [Ca24Al28O64]4+(e)4 is stable at room temperature.[3] In these salts, the electron is delocalized between the cations. Electrides are paramagnetic and Mott insulators. Properties of these salts have been analyzed[4]


Solutions of electride salts are powerful reducing agents, as demonstrated by their use in the Birch reduction. Evaporation of these blue solutions affords a mirror of Na. Such solutions slowly lose their colour as the electrons reduce ammonia:

[Na(NH3)6]+e + NH3NaNH2 + H2

High-pressure elements[edit]

Theoretical evidence supports electride behaviour in insulating high-pressure forms of potassium, sodium, and lithium. Here the isolated electron is stabilized by efficient packing, which reduces enthalpy under external pressure. The electride is identified by a maximum in the electron localization function, which distinguishes the electride from pressure-induced metallization. Electride phases are typically semiconducting or have very low conductivity.[5][6][7]

See also[edit]


  1. ^ Dye, J. L. (2003). "Electrons as Anions". Science. 301 (5633): 607–608. PMID 12893933. doi:10.1126/science.1088103. 
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5
  3. ^ Buchammagari, H.; et al. (2007). "Room Temperature-Stable Electride as a Synthetic Organic Reagent: Application to Pinacol Coupling Reaction in Aqueous Media". Org. Lett. ACS Publications. 9 (21): 4287–4289. PMID 17854199. doi:10.1021/ol701885p. 
  4. ^ Wagner, M. J.; Huang, R. H.; Eglin, J. L.; Dye, J. L. Nature, 1994,368, 726-729.
  5. ^ Marques M.; et al. (2009). "Potassium under Pressure: A Pseudobinary Ionic Compound". Physical Review Letters. 103 (11): 115501. Bibcode:2009PhRvL.103k5501M. PMID 19792381. doi:10.1103/PhysRevLett.103.115501. 
  6. ^ Gatti M.; et al. (2010). "Sodium: A Charge-Transfer Insulator at High Pressures". Physical Review Letters. 104 (11): 216404. Bibcode:2010PhRvL.104u6404G. PMID 20867123. arXiv:1003.0540Freely accessible. doi:10.1103/PhysRevLett.104.216404. 
  7. ^ Marques M.; et al. (2011). "Crystal Structures of Dense Lithium: A Metal-Semiconductor-Metal Transition". Physical Review Letters. 106 (9): 095502. Bibcode:2011PhRvL.106i5502M. PMID 21405633. doi:10.1103/PhysRevLett.106.095502. 

Further reading[edit]