An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of [Na(NH3)6]+ and solvated electrons:
- Na + 6 NH3 → [Na(NH3)6]+,e-
Addition of 2,2,2-cryptand to a solution of [Na(NH3)6]+e− affords [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 [Ca24Al28O68]4+(e-)4 is stable at room temperature. In these salts, the electron is delocalized between the cations. Electrides are paramagnetic and Mott insulators.
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.
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