Electride

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

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]

Reactions

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⁻ + NH₃ → NaNH₂ + H₂

High pressure elements

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]

References

1. Dye, J. L. (2003). "Electrons as Anions". Science 301 (5633): 607–608. doi:10.1126/science.1088103. PMID 12893933.
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. doi:10.1021/ol701885p. PMID 17854199.
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. 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. arXiv:1003.0540. Bibcode:2010PhRvL.104u6404G. 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. doi:10.1103/PhysRevLett.106.095502.

Further reading

• J. L. Dye, M. J. Wagner, G. Overney, R. H. Huang, T. F. Nagy, and D. Tománek (1996). "Cavities and Channels in Electrides" (reprint). J. Am. Chem. Soc. 118 (31): 7329. doi:10.1021/ja960548z.
• JCTC