Hydride

In

Almost all of the elements form binary compounds with hydrogen, the exceptions being He,^[2] Ne,^[3] Ar,^[4] Kr,^[5] Pm, Os, Ir, Rn, Fr, and Ra.^[6][7][8][9] Exotic molecules such as positronium hydride have also been made.

Bonds

Bonds between hydrogen and the other elements range from highly to somewhat covalent. Some hydrides, e.g.

• Hydrides such as
  super hydride, are commonly used as reducing agents in chemical synthesis
  . The hydride adds to an electrophilic center, typically unsaturated carbon.
• Hydrides such as
  Bronsted acid
  releasing H₂.
• Hydrides such as calcium hydride are used as desiccants, 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.^[11]

• 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

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

${\displaystyle {\ce {H- + H+ -> H2}}}$ ΔH = −1676 kJ/mol

The low electron affinity of hydrogen and the strength of the H–H bond (ΔH_BE = 436 kJ/mol) means that the hydride ion would also be a strong reducing agent

${\displaystyle {\ce {H2 + 2e- <=> 2H-}}}$ E^⊖ = −2.25 V

Types of hydrides

According to the general definition, every element of the periodic table (except some noble gases) forms one or more hydrides. These substances have been classified into three main types according to the nature of their bonding:^[6]

• Ionic hydrides, which have significant ionic bonding character.
• Covalent hydrides, which include the hydrocarbons and many other compounds which 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 are composed of hydride bound to an electropositive metal, generally an

${\displaystyle {\ce {NaH + H2O -> H2_{(g)}{}+ NaOH}}}$

ΔH = −83.6 kJ/mol, ΔG = −109.0 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.

${\displaystyle {\ce {{\overset {lithium\\ hydride}{4 LiH}}+ AlCl3 -> LiAlH4{}+ 3 LiCl}}}$

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

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 (NaBH₄) and lithium aluminium hydride and hindered reagents such as DIBAL.

Interstitial hydrides or metallic hydrides

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. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usually 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 cells. 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.^[13]

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.^[14]

Transition metal hydride complexes

Transition metal hydrides include compounds that can be classified as covalent hydrides. Some are even classified as interstitial hydrides^[

, which exists between positively polarized protons and electronegative atoms with open lone pairs.

Protides

Hydrides containing protium are known as protides.

Deuterides

Hydrides containing

LiD, are important fusion fuels in thermonuclear weapons and useful moderators in nuclear reactors

.

Tritides

Hydrides containing tritium are known as tritides.

Mixed anion compounds

oxyhydrides

and others.

Appendix on nomenclature

Protide, deuteride and tritide are used to describe ions or compounds that contain

hydrogen-1, deuterium or tritium

, respectively.

In the classic meaning, hydride refers to any

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:^[9]

• alkali and alkaline earth metals: metal hydride
• boron: borane, BH₃
• aluminium: alumane, AlH₃
• gallium: gallane, GaH₃
• indigane
  , InH₃
• thallium: thallane, TlH₃
• carbon: alkanes, alkenes, alkynes, and all hydrocarbons
• silicon: silane
• germanium: germane
• tin: stannane
• lead: plumbane
• nitrogen: ammonia ("azane" when substituted), hydrazine
• phosphorus: phosphine (note "phosphane" is the IUPAC recommended name)
• arsenic: arsine (note "arsane" is the IUPAC recommended name)
• antimony: stibine (note "stibane" is the IUPAC recommended name)
• bismuth: bismuthine (note "bismuthane" is the IUPAC recommended name)
• helium: helium hydride (only exists as an ion)

According to the convention above, the following are "hydrogen compounds" and not "hydrides":^{[citation needed]}

• oxygen: 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)
• hydrogen polonide
  ("polane" when substituted)
• halogens: hydrogen halides

Examples:

• 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 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
• semiconductors
• stibine: used in semiconductor industry
• phosphine: used for fumigation
• silane: many industrial uses, e.g. manufacture of composite materials 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 hydrocarbons 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

group 15 and group 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".

See also

• Parent hydride
• Hydron (hydrogen cation)
• Hydronium
• Proton
• Hydrogen ion
• Hydride compressor
• Superhydrides

References

1. doi:10.1351/goldbook.H02904
   . Retrieved 11 May 2021.
2. Helium hydride
   exists as an ion.
3. Neonium
   is an ion, and the HNe excimer exists also.
4. ^ Argonium exists as an ion.
5. ^ Kryptonium ion exist as a cation.
6. ^
   OCLC 48138330
   .
7. ISBN 978-81-265-1554-7
   .
8. ISBN 978-0-471-49039-5
   .
9. ^ ^{a b} Nomenclature of Inorganic Chemistry ("The Red Book") (PDF). IUPAC Recommendations. 2005. Par. IR-6.
10. S2CID 49413857
    .
11. PMID 15008624
    .
12. ISBN 0-471-11280-1
    .
13. ^ Palladium hydride
14. doi:10.1016/s1369-7021(03)00922-2
    .
15. doi:10.1039/c39800000295
    .
16. ISBN 0-471-18768-2

Bibliography

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

External links

Look up hydride in Wiktionary, the free dictionary.

• Media related to Hydrides at Wikimedia Commons