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 solid 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 are Mott insulators. Properties of these salts have been analyzed.[4]
Reactions[edit]
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:
- 2[Na(NH3)6]+e− → 2NaNH2 + 10NH3 + H2
This conversion is catalyzed by various metals.[5] An electride, [Na(NH3)6]+e−, is formed as a reaction intermediate.
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,[6][7][8] usually with a complex optical response.[9] A sodium compound called disodium helide has been created under 113 gigapascals (1.12×106 atm) of pressure.[10]
See also[edit]
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