Editing Hydride

{{About|binary compounds of hydrogen with another element|the anion of hydrogen|Hydrogen anion}}{{Short description|Molecule with a hydrogen bound to a more electropositive element or group}}
In [[chemistry]], a '''hydride''' is formally the [[anion]] of [[hydrogen]] (H<sup>&minus;</sup>), a hydrogen ion with two electrons.<ref name="hydron Also contains definitions of: hydride, hydro">{{cite journal |title=hydron (H02904) | website=IUPAC|date=24 February 2014 | doi=10.1351/goldbook.H02904|url=https://goldbook.iupac.org/terms/view/H02904 |access-date=11 May 2021|doi-access=free}}</ref> In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all [[chemical compound|compounds]] containing [[covalent bond|covalently bound]] H [[atom]]s. In this broad and potentially archaic sense, [[water]] (H<sub>2</sub>O) is a hydride of [[oxygen]], [[ammonia]] is a hydride of [[nitrogen]], etc. In covalent compounds, it implies hydrogen is attached to a less [[electronegative]] [[chemical element|element]]. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.

[[File:NaH.jpg|thumb|Sodium hydride as an example of hydride salt]]

Almost all of the elements form [[Binary compounds of hydrogen|binary compounds with hydrogen]], the exceptions being [[helium|He]],<ref>[[Helium hydride]] exists as an ion.</ref> [[neon|Ne]],<ref>[[Neonium]] is an ion, and the HNe excimer exists also.</ref> [[argon|Ar]],<ref>[[Argonium]] exists as an ion.</ref> [[krypton|Kr]],<ref>[[Kryptonium ion]] exist as a cation.</ref> [[promethium|Pm]], [[osmium|Os]], [[iridium|Ir]], [[radon|Rn]], [[francium|Fr]], and [[radium|Ra]].<ref name=Greenwood>{{cite book |last1=Greenwood |first1=N. N. |last2=Earnshaw |first2=A. |title=Chemistry of the elements |publisher=Butterworth-Heinemann |publication-place=Boston, Mass |edition=2nd |year=1997 |isbn=0-7506-3365-4 |oclc=48138330}}</ref><ref name="Lee2008">{{cite book |last=Lee |first=J.D. |title=Concise Inorganic Chemistry |edition=5th |publisher=Wiley |year=2008 |isbn=978-81-265-1554-7 |url=https://books.google.com/books?id=0m6dIGxmAfwC}}</ref><ref name="Massey2000">{{cite book |last=Massey |first=A.G. |title=Main Group Chemistry |publisher=Wiley |series=Inorganic Chemistry |year=2000 |isbn=978-0-471-49039-5 |url=https://books.google.com/books?id=ujUvAQAAIAAJ}}</ref><ref name=redbook2005/> [[exotic atom#exotic molecules|Exotic molecules]] such as [[positronium hydride]] have also been made.

==Bonds==
Bonds between hydrogen and the other elements range from being highly ionic to somewhat covalent<!-- most commentators would say somewhat covalent for the saline hydrides.-->. Some hydrides, e.g. [[Boranes|boron hydrides]], do not conform to classical [[electron counting]] rules and the bonding is described in terms of multi-centered bonds, whereas the interstitial hydrides often involve [[metallic bond]]ing. Hydrides can be discrete [[molecule]]s, [[oligomer]]s or [[polymer]]s, [[ionic solid]]s, [[chemisorption|chemisorbed]] monolayers,{{citation needed|date=April 2014}}<!--example would be nice as would a reference--> bulk metals (interstitial), or other materials.  While hydrides traditionally react as [[Lewis base]]s or [[reducing agent]]s, some metal hydrides behave as hydrogen-atom donors and act as acids.

==Applications==
[[image:TTMSS.png|thumb|right|upright=0.9|[[Tris(trimethylsilyl)silane]] is an example of a hydride with a weak bond to H. It is used as a source of hydrogen atoms.<ref>{{cite journal|title=Thirty Years of (TMS)<sub>3</sub>SiH: A Milestone in Radical-Based Synthetic Chemistry |journal=Chemical Reviews|year=2018|volume=118|issue=14|pages=6516–6572|doi=10.1021/acs.chemrev.8b00109|pmid=29938502|last1=Chatgilialoglu|first1=Chryssostomos|last2=Ferreri|first2=Carla|last3=Landais|first3=Yannick|last4=Timokhin|first4=Vitaliy I.|s2cid=49413857 }}</ref>]]
[[File:Katalysezyklus-Wilkinson.png|upright=1.7|right|thumb|Metal hydrides (e.g. H<sub>2</sub>RhCl(PPh<sub>3</sub>)<sub>2</sub> derived from [[Wilkinson's catalyst]]) are intermediates in hydrogenation catalysis.]]

*Hydrides such as [[sodium borohydride]], [[lithium aluminium hydride]], [[diisobutylaluminium hydride]] (DIBAL) and [[super hydride]], are commonly used as [[reducing agent]]s in [[chemical synthesis]]. The hydride adds to an electrophilic center, typically unsaturated carbon.
*Hydrides such as [[sodium hydride]] and [[potassium hydride]] are used as strong [[Base (chemistry)|base]]s in [[organic synthesis]]. The hydride reacts with the weak [[Bronsted acid]] releasing H<sub>2</sub>.
*Hydrides such as [[calcium hydride]] are used as [[desiccant]]s, i.e. drying agents, to remove trace water from organic solvents. The hydride reacts with water forming [[hydrogen]] and [[hydroxide]] salt. The dry solvent can then be distilled or vacuum transferred from the "solvent pot".
*Hydrides are important in storage battery technologies such as [[nickel-metal hydride battery]]. Various metal hydrides have been examined for use as a means of hydrogen storage for [[fuel cell]]-powered electric cars and other purposed aspects of a [[hydrogen economy]].<ref>{{Cite journal
| doi = 10.1021/cr030691s
| volume = 104
| issue = 3
| pages = 1283–1316
| title = Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen
| journal = Chemical Reviews
| date = 2004-03-01
| pmid = 15008624
| last1 = Grochala
| first1 = Wojciech
| last2 = Edwards
| first2 = Peter P.
}}</ref>
* Hydride complexes are catalysts and catalytic intermediates in  a variety of homogeneous and heterogeneous catalytic cycles. Important examples include [[hydrogenation]], [[hydroformylation]], [[hydrosilylation]], [[hydrodesulfurization]] catalysts. Even certain enzymes, the [[hydrogenase]], operate via hydride intermediates. The energy carrier [[nicotinamide adenine dinucleotide]] reacts as a hydride donor or hydride equivalent.

== Hydride ion ==
{{See also|Hydrogen anion}}
Free hydride anions exist only under extreme conditions and are not invoked for homogeneous solution. Instead, many compounds have hydrogen centres with hydridic character.

Aside from [[electride]], the hydride ion is the simplest possible [[anion]], consisting of two [[electron]]s and a [[proton]]. Hydrogen has a relatively low [[electron affinity]], 72.77&nbsp;kJ/mol and reacts exothermically with protons as a powerful [[Lewis base]]. 

{{block indent|<chem>H- + H+ -> H2</chem>{{pad|3em}}{{math|1=[[Enthalpy|Δ''H'']] = −1676&nbsp;kJ/mol}}}}

The low electron affinity of hydrogen and the strength of the H&ndash;H bond ({{math|1=Δ''H''<sub>BE</sub> = 436&nbsp;kJ/mol}}) means that the hydride ion would also be a strong [[reducing agent]]

{{block indent|<chem>H2 + 2e-  <=>  2H-</chem>{{pad|3em}}{{math|1=[[Standard electrode potential|''E''<sup>⊖</sup>]] = −2.25&nbsp;V}}}}

== Types of hydrides ==
According to the general definition, every element of the [[periodic table]] (except some [[noble gas]]es) forms one or more hydrides. These substances have been classified into three main types according to the nature of their [[Chemical bond|bonding]]:<ref name=Greenwood/>
*''Ionic hydrides'', which have significant [[ionic bonding]] character.
*''Covalent hydrides'', which include the hydrocarbons and many other compounds which [[Covalent bond|covalently bond]] to hydrogen atoms.
*''Interstitial hydrides'', which may be described as having [[metallic bonding]].
While these divisions have not been used universally, they are still useful to understand differences in hydrides.

=== Ionic hydrides ===
These are stoichiometric compounds of hydrogen. Ionic or '''saline hydrides'''<ref name="UllmannH2">{{cite book |doi=10.1002/14356007.a13_297.pub3 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2013 |last1=Lauermann |first1=Gerhard |last2=Häussinger |first2=Peter |last3=Lohmüller |first3=Reiner |last4=Watson |first4=Allan M. |pages=1–15 |isbn=978-3-527-30673-2 }}</ref> are composed of hydride bound to an electropositive metal, generally an [[alkali metal]] or [[alkaline earth metal]]. The divalent [[lanthanide]]s such as [[europium]] and [[ytterbium]] form compounds similar to those of heavier alkaline earth metals. In these materials the hydride is viewed as a [[pseudohalide]]. Saline hydrides are insoluble in conventional solvents, reflecting their non-molecular structures. Ionic hydrides are used as bases and, occasionally, as reducing [[reagent]]s in [[organic synthesis]].<ref>{{cite book|last=Brown|first=H. C.|title=Organic Syntheses via Boranes|url=https://archive.org/details/organicsyntheses0000brow|url-access=registration|publisher=John Wiley & Sons|location=New York|date=1975|isbn=0-471-11280-1}}</ref>

{{block indent|[[Acetophenone|C<sub>6</sub>H<sub>5</sub>C(O)CH<sub>3</sub>]] + [[Potassium hydride|KH]] → C<sub>6</sub>H<sub>5</sub>C(O)CH<sub>2</sub>K + H<sub>2</sub>}}

Typical solvents for such reactions are [[ethers]]. [[Water]] and other [[protic solvent]]s cannot serve as a medium for ionic hydrides because the hydride ion is a stronger [[Base (chemistry)|base]] than [[hydroxide]] and most [[hydroxyl]] anions. Hydrogen gas is liberated in a typical acid-base reaction.

{{block indent|<chem>NaH + H2O -> H2_{(g)}{} + NaOH</chem>}}
{{block indent|1=Δ''H'' = −83.6&nbsp;kJ/mol, [[Gibbs free energy|Δ''G'']] = −109.0&nbsp;kJ/mol}}

Often alkali metal hydrides react with metal halides. [[Lithium aluminium hydride]] (often abbreviated as LAH) arises from reactions of [[lithium hydride]] with [[aluminium chloride]].

{{block indent|4 [[Lithium hydride|LiH]] + AlCl<sub>3</sub> → LiAlH<sub>4</sub> + 3 LiCl}}

===Covalent hydrides===
According to some definitions, covalent hydrides cover all other compounds containing hydrogen. Some definitions limit hydrides to hydrogen centres that formally react as hydrides, i.e. are nucleophilic, and hydrogen atoms bound to metal centers. These hydrides are formed by all the true non-metals (except zero group elements) and the elements like Al, Ga, Sn, Pb, Bi, Po, etc., which are normally metallic in nature, i.e., this class includes the hydrides of p-block elements. In these substances the hydride bond is formally a [[covalent bond]] much like the bond made by a proton in a [[weak acid]]. This category includes hydrides that exist as discrete molecules, polymers or oligomers, and hydrogen that has been chem-adsorbed to a surface.  A particularly important segment of covalent hydrides are [[complex metal hydride]]s, powerful soluble hydrides commonly used in synthetic procedures.

Molecular hydrides often involve additional ligands; for example, [[diisobutylaluminium hydride]] (DIBAL) consists of two aluminum centers bridged by hydride ligands. Hydrides that are soluble in common solvents are widely used in organic synthesis. Particularly common are [[sodium borohydride]] ({{chem2|NaBH4}}) and [[lithium aluminium hydride]] and hindered reagents such as DIBAL.

===Interstitial hydrides or metallic hydrides===
[[File:Metal Hydride for Hydrogen Storage-Ovonic.jpg|thumb|Metal hydride for hydrogen storage applications]]
Interstitial hydrides most commonly exist within metals or alloys. They are traditionally termed "compounds" even though they do not strictly conform to the definition of a compound, more closely resembling common alloys such as steel. In such hydrides, hydrogen can exist as either atomic or diatomic entities. Mechanical or thermal processing, such as bending, striking, or annealing, may cause the hydrogen to precipitate out of solution by degassing. Their bonding is generally considered [[metallic bonding|metallic]]. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usually [[Non-stoichiometric compound|non-stoichiometric]], with variable amounts of hydrogen atoms in the lattice. In materials engineering, the phenomenon of [[hydrogen embrittlement]] results from the formation of interstitial hydrides. Hydrides of this type form according to either one of two main mechanisms. The first mechanism involves the adsorption of dihydrogen, succeeded by the cleaving of the H-H bond, the delocalisation of the hydrogen's electrons, and finally the diffusion of the protons into the metal lattice. The other main mechanism involves the electrolytic reduction of ionised hydrogen on the surface of the metal lattice, also followed by the diffusion of the protons into the lattice. The second mechanism is responsible for the observed temporary volume expansion of certain electrodes used in electrolytic experiments.

[[Palladium]] absorbs up to 900 times its own volume of hydrogen at room temperatures, forming [[palladium hydride]]. This material has been discussed as a means to carry hydrogen for vehicular [[fuel cell]]s. Interstitial hydrides show certain promise as a way for safe [[hydrogen storage]]. Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice (in an fcc lattice there is one octahedral hole per metal atom). The limit of absorption at normal pressures is PdH0.7, indicating that approximately 70% of the octahedral holes are occupied.<ref>[[Palladium hydride]]</ref>

Many interstitial hydrides have been developed that readily absorb and discharge hydrogen at room temperature and atmospheric pressure. They are usually based on [[intermetallic]] compounds and solid-solution alloys. However, their application is still limited, as they are capable of storing only about 2 weight percent of hydrogen, insufficient for automotive applications.<ref>{{cite journal |title=Materials for hydrogen storage|journal= Materials Today|volume= 6|issue= 9|year=2003|pages=24–33|doi=10.1016/s1369-7021(03)00922-2|last1= Züttel|first1= Andreas|doi-access= free}}</ref>
[[File:PAHCRU.png|thumb|Structure of {{chem2|[HRu6(CO)18]-}}, a metal cluster with an interstitial hydride ligand (small turquoise sphere at center).<ref>{{cite journal |title=Direct location of the interstitial hydride ligand in [HRu6(CO)18]– by both X-ray and neutron analyses of [Ph4As][HRu6(CO)18] by Both X-ray and Neutron Analyses of [Ph4As][HRu6(CO)18]|journal=Journal of the Chemical Society, Chemical Communications|issue=7|year=1980|page=295|doi=10.1039/c39800000295|last1=Jackson|first1=Peter F.|last2=Johnson|first2=Brian F. G.|last3=Lewis|first3=Jack|last4=Raithby|first4=Paul R.|last5=McPartlin|first5=Mary|last6=Nelson|first6=William J. H.|last7=Rouse|first7=Keith D.|last8=Allibon|first8=John|last9=Mason|first9=Sax A.}}</ref> ]]

===Transition metal hydride complexes===
{{Main|Transition metal hydride}}
Transition metal hydrides include compounds that can be classified as ''covalent hydrides''. Some are even classified as interstitial hydrides{{citation needed|date=October 2013}} and other bridging hydrides. Classical<!--this is an odd term- classic hydrides has been used elsewhere in wikipedia with a different meaning --> transition metal hydride feature a single bond between the hydrogen centre and the transition metal.  Some transition metal hydrides are acidic, e.g., {{chem2|HCo(CO)4}} and {{chem2|H2Fe(CO)4}}. The anions [[potassium nonahydridorhenate]] {{chem2|[ReH9](2-)}} and {{chem2|[FeH6](4-)}} are examples from the growing collection<!--an example of an old refence bing used to support the changed statemnt it originally saqid rare --> of known molecular [[homoleptic]] metal hydrides.<ref>A. Dedieu (Editor) Transition Metal Hydrides 1991, Wiley-VCH, Weinheim. {{ISBN|0-471-18768-2}}</ref>  As [[pseudohalide]]s, hydride ligands are capable of bonding with positively polarized hydrogen centres. <!-- obscure, over technical language  --> This interaction, called [[dihydrogen bond]]ing, is similar to [[hydrogen bonding]], which exists between positively polarized protons and electronegative atoms with open lone pairs.

=== Protides ===
Hydrides containing [[Isotopes of hydrogen#Hydrogen-1 (Protium)|protium]] are known as ''protides''.

===Deuterides===
Hydrides containing [[deuterium]] are known as ''deuterides''. Some deuterides, such as [[lithium deuteride|LiD]], are important fusion fuels in [[thermonuclear weapon]]s and useful moderators in [[nuclear reactor]]s.

=== Tritides ===
Hydrides containing [[tritium]] are known as ''tritides.''

===Mixed anion compounds===
[[Mixed anion compounds]] exist that contain hydride with other anions. These include boride hydrides, [[carbohydrides]], [[hydridonitrides]], [[oxyhydrides]] and others.

==Appendix on nomenclature==
''Protide'', ''deuteride'' and ''tritide'' are used to describe ions or compounds that contain [[isotopic enrichment|enriched]] [[hydrogen-1]], [[deuterium]] or [[tritium]], respectively.

In the classic meaning, hydride refers to any [[Chemical compound|compound]] hydrogen forms with other elements, ranging over [[Periodic table group|groups]] 1–16 (the [[binary compounds of hydrogen]]). The following is a list of the nomenclature for the hydride derivatives of main group compounds according to this definition:<ref name=redbook2005>{{cite book |url=http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |title=Nomenclature of Inorganic Chemistry ("The Red Book")|series=IUPAC Recommendations |year=2005 |id=Par. IR-6 }}</ref>
*[[alkali metal|alkali]] and [[alkaline earth metal|alkaline earth]] metals: metal hydride
*[[boron]]: [[borane]], BH<sub>3</sub>
*[[aluminium]]: [[aluminium hydride|alumane]], AlH<sub>3</sub>
*[[gallium]]: [[gallane]], GaH<sub>3</sub>
*[[indium]]: [[indigane]], InH<sub>3</sub>
*[[thallium]]: [[thallane]], TlH<sub>3</sub>
*[[carbon]]: [[alkane]]s, [[alkene]]s, [[alkyne]]s, and all [[hydrocarbon]]s
*[[silicon]]: [[silane]]
*[[germanium]]: [[germane]]
*[[tin]]: [[stannane]]
*[[lead]]: [[plumbane]]
*[[nitrogen]]: [[ammonia]] ("azane" when [[substituent|substituted]]), [[hydrazine]]
*[[phosphorus]]: [[phosphine]] (note "phosphane" is the [[IUPAC nomenclature of inorganic chemistry 2005|IUPAC]] recommended name)
*[[arsenic]]: [[arsine]] (note "arsane" is the [[IUPAC nomenclature of inorganic chemistry 2005|IUPAC]] recommended name)
*[[antimony]]: [[stibine]] (note "stibane" is the [[IUPAC nomenclature of inorganic chemistry 2005|IUPAC]] recommended name)
*[[bismuth]]: [[bismuthine]] (note "bismuthane" is the [[IUPAC nomenclature of inorganic chemistry 2005|IUPAC]] recommended name)
*[[helium]]: [[Helium hydride ion|helium hydride]] (only exists as an ion)

According to the convention above, the following are "hydrogen compounds" and not "hydrides":{{Citation needed|date=July 2010}}
*[[oxygen]]: [[Properties of water|water]] ("oxidane" when substituted; synonym: hydrogen oxide), [[hydrogen peroxide]]
*[[sulfur]]: [[hydrogen sulfide]] ("sulfane" when substituted)
*[[selenium]]: [[hydrogen selenide]] ("selane" when substituted)
*[[tellurium]]: [[hydrogen telluride]] ("tellane" when substituted)
*[[polonium]]: [[hydrogen polonide]] ("polane" when substituted)
*[[halogen]]s: hydrogen halides

Examples:
*[[nickel hydride]]: used in [[NiMH battery|NiMH batteries]]
*[[palladium hydride]]: electrodes in [[cold fusion]] experiments
*[[lithium aluminium hydride]]: a powerful reducing agent used in organic chemistry
*[[sodium borohydride]]: selective specialty reducing agent, hydrogen storage in [[direct borohydride fuel cell|fuel cells]]
*[[sodium hydride]]: a powerful base used in organic chemistry
*[[diborane]]: reducing agent, rocket fuel, semiconductor dopant, catalyst, used in organic synthesis; also [[borane]], [[pentaborane]] and [[decaborane]]
*[[arsine]]: used for [[Doping (semiconductor)|doping]] [[semiconductors]]
*[[stibine]]: used in [[semiconductor]] industry
*[[phosphine]]: used for [[fumigation]]
*[[silane]]: many industrial uses, e.g. manufacture of [[composite material]]s and water repellents
*[[ammonia]]: [[coolant]], [[fuel]], [[fertilizer]], many other industrial uses
*[[hydrogen sulfide]]: component of [[natural gas]], important source of [[sulfur]]
*Chemically, even [[water]] and [[hydrocarbon]]s could be considered hydrides.

All metalloid hydrides are highly flammable. All solid non-metallic hydrides except [[ice]] are highly flammable. But when hydrogen combines with halogens it produces acids rather than hydrides, and they are not flammable.

===Precedence convention===
According to [[IUPAC inorganic nomenclature|IUPAC convention]], by precedence (stylized electronegativity), hydrogen falls between [[nitrogen group|group 15]] and [[chalcogen|group 16]] elements. Therefore, we have NH<sub>3</sub>, "nitrogen hydride" (ammonia), versus H<sub>2</sub>O, "hydrogen oxide" (water). This convention is sometimes broken for polonium, which on the grounds of polonium's metallicity is often referred to as "polonium hydride" instead of the expected "hydrogen polonide".

==See also==
*[[Parent hydride]]
*[[Hydron (chemistry)|Hydron]] (hydrogen cation)
*[[Hydronium]]
*[[Proton]]
*[[Hydrogen ion]]
*[[Hydride compressor]]
*[[Superhydrides]]

==References==
{{Reflist}}

==Bibliography==
* W. M. Mueller, J. P. Blackledge, G. G. Libowitz, ''Metal Hydrides'', Academic Press, N.Y. and London, (1968)

==External links==
{{Wiktionary|hydride}}
*{{Commons category-inline|Hydrides}}

{{Monatomic anion compounds}}
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[[Category:Hydrides| ]]
[[Category:Anions]]
[[Category:Hydrogen storage]]
[[Category:Functional groups]]