An electride is an

ionic compound In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions (Cation, cations) and negatively charged ions (Anion, anions), which results in a compound with no net electric charge (electrica ...

in which an

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

serves the role of the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

Solutions

Solutions of

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

s in

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

are electride salts. In the case of sodium, these blue solutions consist of a(NH₃)₆sup>+ and solvated electrons: :Na + 6 NH₃ → a(NH₃)₆sup>+ + e⁻ The cation a(NH₃)₆sup>+ is an octahedral

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

. 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 .2.2cryptand to a solution of a(NH₃)₆sup>+e⁻ affords a (crown ether)sup>+e⁻ or a(2,2,2-crypt)sup>+e⁻. Evaporation of these solutions yields a blue-black paramagnetic solid with the formula a(2,2,2-crypt)sup>+e⁻. Most solid electride salts decompose above 240 K, although a₂₄Al₂₈O₆₄sup>4+(e⁻)₄ is stable at room temperature. In these salts, the electron is delocalized between the

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s. Properties of these salts have been analyzed. ThI₂ and ThI₃ have also been proposed to be electride compounds. Similarly, , , , and are all electride salts with a tricationic metal ion.

Organometallic electrides

Magnesium reduced nickel(II)-bipyridyl (bipy) complex have been labeled organic electrides. An example is THF)₄Mg₄(μ²-bipy)₄sup>–, 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

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

s, 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 a(NH₃)₆sup>+e⁻ → 2 NaNH₂ + 10NH₃ + H₂ This conversion is catalyzed by various metals. An electride, a(NH₃)₆sup>+e⁻, is formed as a

reaction intermediate In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding interme ...

High-pressure elements

quantum chemistry Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...

, 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

), 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.

Solutions

Solid salts

Organometallic electrides

Reactions

High-pressure elements

Layered electrides (Electrenes)

See also

References

Further reading