Inchemistry, ahydride is formally theanion ofhydrogen (H−), a hydrogen ion with two electrons.[1] 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 allcompounds containingcovalently bound Hatoms. In this broad and potentially archaic sense,water (H2O) is a hydride ofoxygen,ammonia is a hydride ofnitrogen, etc. In covalent compounds, it implies hydrogen is attached to a lesselectronegativeelement. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.
Almost all of the elements formbinary compounds with hydrogen, the exceptions beingHe,[2]Ne,[3]Ar,[4]Kr,[5]Pm,Os,Ir,Rn,Fr, andRa.[6][7][8][9]Exotic molecules such aspositronium hydride have also been made.
Bonds between hydrogen and the other elements range from being highly ionic to somewhat covalent. Some hydrides, e.g.boron hydrides, do not conform to classicalelectron counting rules and the bonding is described in terms of multi-centered bonds, whereas the interstitial hydrides often involvemetallic bonding. Hydrides can be discretemolecules,oligomers orpolymers,ionic solids,chemisorbed monolayers,[citation needed] bulk metals (interstitial), or other materials. While hydrides traditionally react asLewis bases orreducing agents, some metal hydrides behave as hydrogen-atom donors and act as acids.
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 fromelectride, the hydride ion is the simplest possibleanion, consisting of twoelectrons and aproton. Hydrogen has a relatively lowelectron affinity, 72.77 kJ/mol and reacts exothermically with protons as a powerfulLewis base.
The low electron affinity of hydrogen and the strength of the H–H bond (ΔHBE = 436 kJ/mol) means that the hydride ion would also be a strongreducing agent
According to the general definition, every element of theperiodic table (except somenoble gases) forms one or more hydrides. These substances have been classified into three main types according to the nature of theirbonding:[6]
While these divisions have not been used universally, they are still useful to understand differences in hydrides.
These are stoichiometric compounds of hydrogen. Ionic orsaline hydrides[12] are composed of hydride bound to an electropositive metal, generally analkali metal oralkaline earth metal. The divalentlanthanides such aseuropium andytterbium form compounds similar to those of heavier alkaline earth metals. In these materials the hydride is viewed as apseudohalide. Saline hydrides are insoluble in conventional solvents, reflecting their non-molecular structures. Ionic hydrides are used as bases and, occasionally, as reducingreagents inorganic synthesis.[13]
Typical solvents for such reactions areethers.Water and otherprotic solvents cannot serve as a medium for ionic hydrides because the hydride ion is a strongerbase thanhydroxide and mosthydroxyl anions. Hydrogen gas is liberated in a typical acid-base reaction.
Often alkali metal hydrides react with metal halides.Lithium aluminium hydride (often abbreviated as LAH) arises from reactions oflithium hydride withaluminium chloride.
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 acovalent bond much like the bond made by a proton in aweak 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 arecomplex metal hydrides, 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 aresodium borohydride (NaBH4) andlithium aluminium hydride and hindered reagents such as DIBAL.
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 consideredmetallic. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usuallynon-stoichiometric, with variable amounts of hydrogen atoms in the lattice. In materials engineering, the phenomenon ofhydrogen 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, formingpalladium hydride. This material has been discussed as a means to carry hydrogen for vehicularfuel cells. Interstitial hydrides show certain promise as a way for safehydrogen 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.[14]
Many interstitial hydrides have been developed that readily absorb and discharge hydrogen at room temperature and atmospheric pressure. They are usually based onintermetallic 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.[15]
Transition metal hydrides include compounds that can be classified ascovalent hydrides. Some are even classified as interstitial hydrides[citation needed] and other bridging hydrides. Classical transition metal hydride feature a single bond between the hydrogen centre and the transition metal. Some transition metal hydrides are acidic, e.g.,HCo(CO)4 andH2Fe(CO)4. The anionspotassium nonahydridorhenate[ReH9]2− and[FeH6]4− are examples from the growing collection of known molecularhomoleptic metal hydrides.[17] Aspseudohalides, hydride ligands are capable of bonding with positively polarized hydrogen centres. This interaction, calleddihydrogen bonding, is similar tohydrogen bonding, which exists between positively polarized protons and electronegative atoms with open lone pairs.
Hydrides containingprotium are known asprotides.
Hydrides containingdeuterium are known asdeuterides. Some deuterides, such asLiD, are important fusion fuels inthermonuclear weapons and useful moderators innuclear reactors.
Hydrides containingtritium are known astritides.
Mixed anion compounds exist that contain hydride with other anions. These include boride hydrides,carbohydrides,hydridonitrides,oxyhydrides and others.
Protide,deuteride andtritide are used to describe ions or compounds that containenrichedhydrogen-1,deuterium ortritium, respectively.
In the classic meaning, hydride refers to anycompound hydrogen forms with other elements, ranging overgroups 1–16 (thebinary compounds of hydrogen). The following is a list of the nomenclature for the hydride derivatives of main group compounds according to this definition:[9]
According to the convention above, the following are "hydrogen compounds" and not "hydrides":[citation needed]
Examples:
All metalloid hydrides are highly flammable. All solid non-metallic hydrides exceptice are highly flammable. But when hydrogen combines with halogens it produces acids rather than hydrides, and they are not flammable.
According toIUPAC convention, by precedence (stylized electronegativity), hydrogen falls betweengroup 15 andgroup 16 elements. Therefore, we have NH3, "nitrogen hydride" (ammonia), versus H2O, "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".
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