electride

Image:Electride 01.jpg

An electride is an ionic compound in which an electron serves the role of the anion.{{cite journal |author=Dye, J. L. |title=Electrons as Anions |journal=Science |year=2003 |volume=301 |pages=607–608 |doi=10.1126/science.1088103 |pmid=12893933 |issue=5633 |s2cid=93768664}}

Solutions

Solutions of alkali metals in ammonia are electride salts.Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}} 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. 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.{{Cite journal |last1=Zurek |first1=Eva |last2=Edwards |first2=Peter P. |last3=Hoffmann |first3=Roald |date=2009-10-19 |title=A Molecular Perspective on Lithium–Ammonia Solutions |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200900373 |journal=Angewandte Chemie International Edition |language=en |volume=48 |issue=44 |pages=8198–8232 |doi=10.1002/anie.200900373 |issn=1433-7851|url-access=subscription }}

Similar solutions exist in hexamethylphosphoramide.{{Cite journal |last=Ellis |first=John E. |date=2006-04-17 |title=Adventures with Substances Containing Metals in Negative Oxidation States |url=https://pubs.acs.org/doi/10.1021/ic052110i |journal=Inorganic Chemistry |language=en |volume=45 |issue=8 |pages=3174-3175 |doi=10.1021/ic052110i |issn=0020-1669}}

Solid salts

Many "inorganic electrides" have been described.{{cite journal |doi=10.1021/acs.chemrev.0c01071 |title=Advances in Materials and Applications of Inorganic Electrides |date=2021 |last1=Hosono |first1=Hideo |last2=Kitano |first2=Masaaki |journal=Chemical Reviews |volume=121 |issue=5 |pages=3121–3185 |pmid=33606511 }}

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.{{cite journal |author=Buchammagari, H. |year=2007 |title=Room Temperature-Stable Electride as a Synthetic Organic Reagent: Application to Pinacol Coupling Reaction in Aqueous Media |journal=Org. Lett. |volume=9 |issue=21 |pages=4287–4289 |doi=10.1021/ol701885p |pmid= 17854199|display-authors=etal}} In these salts, the electron is delocalized between the cations. Properties of these salts have been analyzed.{{cite journal |author=Wagner, M. J.; Huang, R. H.; Eglin, J. L.; Dye, J. L. |year=1994 |title=An electride with a large six-electron ring |journal=Nature |volume=368 |issue=6473 |pages=726–729 |doi=10.1038/368726a0 |bibcode=1994Natur.368..726W |s2cid=4242499}}.

ThI₂ and ThI₃ have also been proposed to be electride compounds.{{cite book |last1=Wickleder |first1=Mathias S. |first2=Blandine |last2=Fourest |first3=Peter K. |last3=Dorhout |ref=Wickleder et al. |contribution=Thorium |title=The Chemistry of the Actinide and Transactinide Elements |editor1-first=Lester R. |editor1-last=Morss |editor2-first=Norman M. |editor2-last=Edelstein |editor3-first=Jean |editor3-last=Fuger |edition=3rd |date=2006 |volume=3 |publisher=Springer |location=Dordrecht, the Netherlands |pages=78–94 |url=http://radchem.nevada.edu/classes/rdch710/files/thorium.pdf |doi=10.1007/1-4020-3598-5_3 |isbn=978-1-4020-3555-5 |url-status=dead |archiveurl=https://web.archive.org/web/20160307160941/http://radchem.nevada.edu/classes/rdch710/files/Thorium.pdf |archivedate=2016-03-07}} Similarly, Cerium#Chemistry, {{chem|La|I|2}}, {{chem|Gd|I|2}}, and {{chem|Pr|I|2}} are all electride salts with a tricationic metal ion.{{Greenwood&Earnshaw2nd|pages=1240-2}}{{cite journal |author=Nief, F. |title=Non-classical divalent lanthanide complexes |journal=Dalton Trans. |year=2010 |volume=39 |issue=29 |pages=6589–6598 |doi=10.1039/c001280g |pmid=20631944}}

= Organometallic 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 is the singly occupied molecular orbital (SOMO) formed by the Mg-square cluster within the larger complex.{{cite journal|last1=Day|first1=Craig S.

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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.{{Cite book |author1=Greenlee, K. W. |author2=Henne, A. L. |chapter=Sodium Amide |title=Inorganic Syntheses |year=1946 |volume=2 |pages=128–135 |doi=10.1002/9780470132333.ch38 |isbn=9780470132333}} An electride, [Na(NH₃)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.{{Cite journal |last1=Postils |first1=Verònica |last2=Garcia-Borràs |first2=Marc |last3=Solà |first3=Miquel |last4=Luis |first4=Josep M. |last5=Matito |first5=Eduard |date=2015-03-05 |title=On the existence and characterization of molecular electrides |url=https://pubs.rsc.org/en/content/articlelanding/2015/cc/c5cc00215j |journal=Chemical Communications |language=en |volume=51 |issue=23 |pages=4865–4868 |doi=10.1039/C5CC00215J |issn=1364-548X|url-access=subscription }} Electride phases are typically semiconducting or have very low conductivity,{{cite journal |author=Marques M. |year=2009 |title=Potassium under Pressure: A Pseudobinary Ionic Compound

|journal=Physical Review Letters |volume=103 |issue=11 |page=115501 |doi=10.1103/PhysRevLett.103.115501 |bibcode=2009PhRvL.103k5501M |display-authors=etal |pmid=19792381}}{{cite journal |author=Gatti M. |year=2010 |title=Sodium: A Charge-Transfer Insulator at High Pressures

|journal=Physical Review Letters |volume=104 |issue=11 |page=216404 |doi=10.1103/PhysRevLett.104.216404 |arxiv=1003.0540 |bibcode=2010PhRvL.104u6404G |display-authors=etal |pmid=20867123 |s2cid=18359072}}{{cite journal |author=Marques M. |year=2011 |title=Crystal Structures of Dense Lithium: A Metal-Semiconductor-Metal Transition |journal=Physical Review Letters |volume=106 |issue=9 |page=095502 |doi=10.1103/PhysRevLett.106.095502 |bibcode=2011PhRvL.106i5502M |display-authors=etal |pmid=21405633 |url=http://discovery.ucl.ac.uk/1301752/1/1301752.pdf}} usually with a complex optical response.{{Cite journal |last1=Yu |first1=Zheng |last2=Geng |first2=Hua Y. |last3=Sun |first3=Y. |last4=Chen |first4=Y. |year=2018 |title=Optical properties of dense lithium in electride phases by first-principles calculations |journal=Scientific Reports |volume=8 |issue=1 |pages=3868 |doi=10.1038/s41598-018-22168-1 |pmid=29497122 |pmc=5832767 |arxiv=1803.05234 |bibcode=2018NatSR...8.3868Y}} A sodium compound called disodium helide has been created under {{convert|113|GPa|e6atm}} of pressure.{{Cite journal |last1=Wang |first1=Hui-Tian |last2=Boldyrev |first2=Alexander I. |last3=Popov |first3=Ivan A. |last4=Konôpková |first4=Zuzana |last5=Prakapenka |first5=Vitali B. |last6=Zhou |first6=Xiang-Feng |last7=Dronskowski |first7=Richard |last8=Deringer |first8=Volker L. |last9=Gatti |first9=Carlo |date=May 2017 |title=A stable compound of helium and sodium at high pressure |journal=Nature Chemistry |volume=9 |issue=5 |pages=440–445 |doi=10.1038/nchem.2716 |pmid=28430195 |issn=1755-4349 |arxiv=1309.3827 |bibcode=2017NatCh...9..440D |s2cid=20459726}} 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.{{Cite journal |last1=Racioppi |first1=Stefano |last2=Storm |first2=Christian V. |last3=McMahon |first3=Malcolm I. |last4=Zurek |first4=Eva |date=2023-11-27 |title=On the Electride Nature of Na-hP4 |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.202310802 |journal=Angewandte Chemie International Edition |language=en |volume=62 |issue=48 |pages=e202310802 |doi=10.1002/anie.202310802 |pmid=37796438 |issn=1433-7851|arxiv=2311.01601 }}{{Cite journal |last1=Neaton |first1=J. B. |last2=Ashcroft |first2=N. W. |date=2001-03-26 |title=On the Constitution of Sodium at Higher Densities |url=https://link.aps.org/doi/10.1103/PhysRevLett.86.2830 |journal=Physical Review Letters |volume=86 |issue=13 |pages=2830–2833 |doi=10.1103/PhysRevLett.86.2830|arxiv=cond-mat/0012123 }}

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),{{Cite journal |last1=Zhang |first1=Leilei |last2=Geng |first2=Hua Y. |last3=Wu |first3=Q. |date=2021-04-16 |title=Prediction of anomalous LA-TA splitting in electrides |url=https://doi.org/10.1063/5.0043276 |journal=Matter and Radiation at Extremes |volume=6 |issue=3 |pages=038403 |doi=10.1063/5.0043276 |issn=2468-2047|arxiv=2104.13151 }} the universal but robust gapless surface state in insulating electride that forming a de facto real space topological distribution of charge carriers,{{Cite journal |last1=Wang |first1=Dan |last2=Song |first2=Hongxing |last3=Zhang |first3=Leilei |last4=Wang |first4=Hao |last5=Sun |first5=Yi |last6=Wu |first6=Fengchao |last7=Chen |first7=Ying |last8=Chen |first8=Xiangrong |last9=Geng |first9=Hua Y. |date=2024-02-01 |title=Universal Metallic Surface States in Electrides |url=https://pubs.acs.org/doi/10.1021/acs.jpcc.3c07496 |journal=The Journal of Physical Chemistry C |language=en |volume=128 |issue=4 |pages=1845–1854 |doi=10.1021/acs.jpcc.3c07496 |issn=1932-7447|arxiv=2402.15798 }} and the colossal charge state of some impurities in them.{{Cite journal |last1=Zhang |first1=Leilei |last2=Wu |first2=Qiang |last3=Li |first3=Shourui |last4=Sun |first4=Yi |last5=Yan |first5=Xiaozhen |last6=Chen |first6=Ying |last7=Geng |first7=Hua Y. |date=2021-02-10 |title=Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium |url=https://pubs.acs.org/doi/10.1021/acsami.0c17095 |journal=ACS Applied Materials & Interfaces |language=en |volume=13 |issue=5 |pages=6130–6139 |doi=10.1021/acsami.0c17095 |issn=1944-8244|arxiv=2103.07605 }}

Layered electrides (Electrenes)

Layered electrides or electrenes are single-layer materials consisting of alternating atomically thin two-dimensional layers of electrons and ionized atoms.{{cite journal |last1=Druffel |first1=Daniel L. |last2=Kuntz |first2=Kaci L. |last3=Woomer |first3=Adam H. |last4=Alcorn |first4=Francis M. |last5=Hu |first5=Jun |last6=Donley |first6=Carrie L. |last7=Warren |first7=Scott C. |title=Experimental Demonstration of an Electride as a 2D Material |journal=Journal of the American Chemical Society |date=2016 |volume=138 |issue=49 |pages=16089–16094 |doi=10.1021/jacs.6b10114 |pmid=27960319 |arxiv=1706.02774 |s2cid=19062953 |url=https://pubs.acs.org/doi/10.1021/jacs.6b10114 |access-date=12 October 2021}}{{cite journal |last1=Druffel |first1=Daniel L. |last2=Woomer |first2=Adam H. |last3=Kuntz |first3=Kaci L. |last4=Pawlik |first4=Jacob T. |last5=Warren |first5=Scott C. |title=Electrons on the surface of 2D materials: from layered electrides to 2D electrenes |journal=Journal of Materials Chemistry C |date=2017 |volume=5 |issue=43 |pages=11196–11213 |doi=10.1039/C7TC02488F |url=https://pubs.rsc.org/en/content/articlelanding/2017/tc/c7tc02488f |access-date=11 October 2021|url-access=subscription }} 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.