Editing Hydride (section)

== 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.