Electride

An electride is an ionic compound in which an electron serves the role of 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−

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 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. In these salts, the electron is delocalized between the cations. Electrides are paramagnetic, and are Mott insulators. Properties of these salts have been analyzed.

ThI2 and ThI3 have also been reported to be electride compounds. Similarly, Cerium,, , and are all electride salts with a tricationic metal ion.

Other kinds of electrides
Magnesium reduced nickel(II)-bipyridyl (bipy) complex have been labeled organic electrides. An example is [(THF)4Mg4(μ2-bipy)4]–, in which the electride ion is centered within a square formed by four Mg2+ centers within the larger complex.

"Inorganic electrides" have also been described.

Reactions
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 metal. If not evaporated, 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. An electride, [Na(NH3)6]+e−, is formed as a reaction intermediate.

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 both the electron localization function and the charge density distribution, which distinguishes the electride from pressure-induced metallization. Electride phases are typically semiconducting or have very low conductivity,  usually with a complex optical response. A sodium compound called disodium helide has been created under 113 GPa of pressure.

The intrinsic polarization between atomic nucleus and the electron anion in these high pressure electrides will lead to unique properties, such as the splitting of the longitudinal and transverse acoustic modes (i.e., LA-TA splitting, an analogue to the LO-TO splitting in ionic compound), the universal but robust gapless surface state in insulating electride that forming a de facto real space topological distribution of charge carriers, and the colossal charge state of some impurities in them.

Layered electrides (Electrenes)
Layered electrides or electrenes are single-layer materials consisting of alternating atomically thin two-dimensional layers of electrons and ionized atoms. The first example was Ca2N, in which the charge (+4) of two calcium ions is balanced by the charge of a nitride ion (-3) in the ion layer plus a charge (-1) in the electron layer.