An electride is an ionic compound in which an electron serves the role of the anion.
Solutions
Solutions of
in
ammonia are electride salts.
[Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ] In the case of sodium, these blue solutions consist of Na(NH
3)
6+ and solvated electrons:
- Na + 6 NH3 → Na(NH3)6+ + e−
The cation Na(NH
3)
6+ is an octahedral coordination complex. Despite the name, the electron does not leave the sodium-ammonia complex, but it is transferred from Na to the vacant orbitals of the coordinated ammonia molecules.
Similar solutions exist in hexamethylphosphoramide.
Solid salts
Many "inorganic electrides" have been described.
Addition of a complexant like crown ether or [2.2.2-Cryptand|[2.2.2-cryptand]] to a solution of Na(NH3)6+e− affords Na+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 Ca24Al28O644+(e−)4 is stable at room temperature.[.]
ThI2 and ThI3 have also been proposed to be electride compounds.
Organometallic electrides
Magnesium reduced nickel(II)-bipyridyl (bipy) complex have been labeled organic electrides. An example is (THF)
4Mg
4(μ
2-bipy)
4–, in which the electride is the singly occupied molecular orbital (SOMO) formed by the Mg-square cluster within the larger complex.
Reactions
Electride salts are powerful
, 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:
- 2Na(NH3)6+e− → 2Sodium amide + 10NH3 + H2
This conversion is catalyzed by various metals.
An electride, Na(NH
3)
6+e
−, is formed as a reaction intermediate.
High-pressure elements
In quantum chemistry, an electride is identified by a maximum of the electron density, characterized by a non-nuclear attractor, a large and negative Laplacian at the critical point, and an Electron Localization Function isosurface close to 1.
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 of pressure.
It has been proven that the localized electron density in high-pressure electrides does not correspond to isolated electrons, but that it is generated by the formation of (multicenter) chemical bonds.
The intrinsic polarization between atomic nucleus and the electron anion in these high pressure electrides can 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),
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 Ca
2N, 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.
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See also
Further reading
