Last updated

In chemistry, a hydride is formally the anion of hydrogen (H). [1] The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative 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.


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 between hydrogen and the other elements range from highly to somewhat covalent. Some hydrides, e.g. 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 bonding. Hydrides can be discrete molecules, oligomers or polymers, ionic solids, chemisorbed monolayers,[ citation needed ] bulk metals (interstitial), or other materials. While hydrides traditionally react as Lewis bases or reducing agents, some metal hydrides behave as hydrogen-atom donors and act as acids.


Tris(trimethylsilyl)silane is an example of a hydride with a weak bond to H. It is used as a source of hydrogen atoms. TTMSS.png
Tris(trimethylsilyl)silane is an example of a hydride with a weak bond to H. It is used as a source of hydrogen atoms.
Metal hydrides (e.g. H2RhCl(PPh3)2 derived from Wilkinson's catalyst) are intermediates in hydrogenation catalysis. Katalysezyklus-Wilkinson.png
Metal hydrides (e.g. H2RhCl(PPh3)2 derived from Wilkinson's catalyst) are intermediates in hydrogenation catalysis.

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 electride, the hydride ion is the simplest possible anion, consisting of two electrons and a proton. Hydrogen has a relatively low electron affinity, 72.77 kJ/mol and reacts exothermically with protons as a powerful Lewis base.

  ΔH = −1676 kJ/mol

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

  Eo = −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]

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 alkali metal or alkaline earth metal. The divalent lanthanides 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 reagents in organic synthesis. [12]

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

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.

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

Interstitial hydrides or metallic hydrides

Metal hydride for hydrogen storage applications Metal Hydride for Hydrogen Storage-Ovonic.jpg
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. 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]

Structure of
[HRu6(CO)18], a metal cluster with an interstitial hydride ligand (small turquoise sphere at center). PAHCRU.png
Structure of [HRu6(CO)18], a metal cluster with an interstitial hydride ligand (small turquoise sphere at center).

Transition metal hydride complexes

Transition metal hydrides include compounds that can be classified as covalent 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 and H2Fe(CO)4. The anions potassium nonahydridorhenate [ReH9]2− and [FeH6]4− are examples from the growing collection of known molecular homoleptic metal hydrides. [16] As pseudohalides, hydride ligands are capable of bonding with positively polarized hydrogen centres. This interaction, called dihydrogen bonding, is similar to hydrogen bonding, which exists between positively polarized protons and electronegative atoms with open lone pairs.


Hydrides containing protium are known as protides.


Hydrides containing deuterium are known as deuterides. Some deuterides, such as LiD, are important fusion fuels in thermonuclear weapons and useful moderators in nuclear reactors.


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 enriched hydrogen-1, deuterium or tritium, respectively.

In the classic meaning, hydride refers to any compound hydrogen forms with other elements, ranging over 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]

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


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 convention, by precedence (stylized electronegativity), hydrogen falls between 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

Related Research Articles

<span class="mw-page-title-main">Alkali metal</span> Group of highly reactive chemical elements

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.

<span class="mw-page-title-main">Chemical bond</span> Lasting attraction between atoms that enables the formation of chemical compounds

A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of electrons as in covalent bonds. The strength of chemical bonds varies considerably; there are "strong bonds" or "primary bonds" such as covalent, ionic and metallic bonds, and "weak bonds" or "secondary bonds" such as dipole–dipole interactions, the London dispersion force, and hydrogen bonding. Strong chemical bonding arises from the sharing or transfer of electrons between the participating atoms.

<span class="mw-page-title-main">Carboxylic acid</span> Organic compound containing a –C(=O)OH group

In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

<span class="mw-page-title-main">Functional group</span> Set of atoms in a molecule which augment its chemical and/or physical properties

In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition. This enables systematic prediction of chemical reactions and behavior of chemical compounds and the design of chemical synthesis. The reactivity of a functional group can be modified by other functional groups nearby. Functional group interconversion can be used in retrosynthetic analysis to plan organic synthesis.

<span class="mw-page-title-main">Hydroxide</span> Chemical compound

Hydroxide is a diatomic anion with chemical formula OH. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

<span class="mw-page-title-main">Ionic bonding</span> Chemical bonding involving attraction between ions

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. It is one of the main types of bonding, along with covalent bonding and metallic bonding. Ions are atoms with an electrostatic charge. Atoms that gain electrons make negatively charged ions. Atoms that lose electrons make positively charged ions. This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complex nature, e.g. molecular ions like NH+
or SO2−
. In simpler words, an ionic bond results from the transfer of electrons from a metal to a non-metal in order to obtain a full valence shell for both atoms.

<span class="mw-page-title-main">Ammonium</span> Polyatomic ion (NH₄, charge +1)

The ammonium cation is a positively-charged polyatomic ion with the chemical formula NH+4 or [NH4]+. It is formed by the protonation of ammonia. Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic groups.

<span class="mw-page-title-main">Haloalkane</span> Group of chemical compounds derived from alkanes containing one or more halogens

The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

<span class="mw-page-title-main">Ionic compound</span> Chemical compound involving ionic bonding

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl) in sodium chloride, or polyatomic species such as the ammonium (NH+
) and carbonate (CO2−
) ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network. Ionic compounds usually form crystalline structures when solid.

<span class="mw-page-title-main">Lewis acids and bases</span> Chemical bond theory

A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane (Me3B) is a Lewis acid as it is capable of accepting a lone pair. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. In the context of a specific chemical reaction between NH3 and Me3B, a lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3. The terminology refers to the contributions of Gilbert N. Lewis.

A chemical nomenclature is a set of rules to generate systematic names for chemical compounds. The nomenclature used most frequently worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC).

In chemistry, a Zintl phase is a product of a reaction between a group 1 or group 2 and main group metal or metalloid. It is characterized by intermediate metallic/ionic bonding. Zintl phases are a subgroup of brittle, high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors.

Complex metal hydrides are salts wherein the anions contain hydrides. In the older chemical literature as well as contemporary materials science textbooks, a "metal hydride" is assumed to be nonmolecular, i.e. three-dimensional lattices of atomic ions. In such systems, hydrides are often interstitial and nonstoichiometric, and the bonding between the metal and hydrogen atoms is significantly ionic. In contrast, complex metal hydrides typically contain more than one type of metal or metalloid and may be soluble but invariably react with water. They exhibit ionic bonding between a positive metal ion with molecular anions containing the hydride. In such materials the hydrogen is bonded with significant covalent character to the second metal or metalloid atoms.

This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant amount of jargon.

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some of them are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H-like character (e.g., ZnH2).

Zinc hydride is an inorganic compound with the chemical formula ZnH2. It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the binary first row transition metal hydrides. A variety of coordination compounds containing Zn–H bonds are used as reducing agents, however ZnH2 itself has no common applications.

<span class="mw-page-title-main">Chemical compound</span> Substance composed of multiple elements that are chemically bonded

A chemical compound is a chemical substance composed of many identical molecules containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element is therefore not a compound. A compound can be transformed into a different substance by a chemical reaction, which may involve interactions with other substances. In this process, bonds between atoms may be broken and/or new bonds formed.

Germyl, trihydridogermanate(1-), trihydrogermanide, trihydridogermyl or according to IUPAC Red Book: germanide is an anion containing germanium bounded with three hydrogens, with formula GeH−3. Germyl is the IUPAC term for the –GeH3 group. For less electropositive elements the bond can be considered covalent rather than ionic as "germanide" indicates. Germanide is the base for germane when it loses a proton.

<span class="mw-page-title-main">Nitrogen pentahydride</span> Chemical compound

Nitrogen pentahydride, also known as ammonium hydride is a hypothetical compound with the chemical formula NH5. There are two theoretical structures of nitrogen pentahydride. One structure is trigonal bipyramidal molecular geometry type NH5 molecule. Its nitrogen atom and hydrogen atoms are covalently bounded, and its symmetry group is D3h. Another predicted structure of nitrogen pentahydride is an ionic compound, make up of an ammonium ion and a hydride ion (NH4+H). Until now, no one has synthesized this substance, or proved its existence, and related experiments have not directly observed nitrogen pentahydride. It is only speculated that it may be a reactive intermediate based on reaction products. Theoretical calculations show this molecule is thermodynamically unstable. The reason might be similar to the instability of nitrogen pentafluoride, so the possibility of its existence is low. However, nitrogen pentahydride might exist in special conditions or high pressure. Nitrogen pentahydride was considered for use as a solid rocket fuel for research in 1966.

Hydrogen compounds are compounds containg the element hydrogen. In these compounds, hydrogen can form in the +1 and -1 oxidation states. Hydrogen can form compounds both ionically and in covalent substances. It is a part of many organic compounds such as hydrocarbons as well as water and other organic substances. The H+ ion is often called a proton because it has one proton and no electrons, although the proton does not move freely. Brønsted–Lowry acids are capable of donating H+ ions to bases.


  1. "hydron (H02904)". IUPAC. 24 February 2014. 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. 1 2 Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the elements (2nd ed.). Boston, Mass: Butterworth-Heinemann. ISBN   0-7506-3365-4. OCLC   48138330.
  7. Lee, J.D. (2008). Concise Inorganic Chemistry (5th ed.). Wiley. ISBN   978-81-265-1554-7.
  8. Massey, A.G. (2000). Main Group Chemistry. Inorganic Chemistry. Wiley. ISBN   978-0-471-49039-5.
  9. 1 2 Nomenclature of Inorganic Chemistry ("The Red Book") (PDF). IUPAC Recommendations. 2005. Par. IR-6.
  10. Chatgilialoglu, Chryssostomos; Ferreri, Carla; Landais, Yannick; Timokhin, Vitaliy I. (2018). "Thirty Years of (TMS)3SiH: A Milestone in Radical-Based Synthetic Chemistry". Chemical Reviews. 118 (14): 6516–6572. doi:10.1021/acs.chemrev.8b00109. PMID   29938502. S2CID   49413857.
  11. Grochala, Wojciech; Edwards, Peter P. (2004-03-01). "Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen". Chemical Reviews. 104 (3): 1283–1316. doi:10.1021/cr030691s. PMID   15008624.
  12. Brown, H. C. (1975). Organic Syntheses via Boranes . New York: John Wiley & Sons. ISBN   0-471-11280-1.
  13. Palladium hydride
  14. Züttel, Andreas (2003). "Materials for hydrogen storage". Materials Today. 6 (9): 24–33. doi: 10.1016/s1369-7021(03)00922-2 .
  15. Jackson, Peter F.; Johnson, Brian F. G.; Lewis, Jack; Raithby, Paul R.; McPartlin, Mary; Nelson, William J. H.; Rouse, Keith D.; Allibon, John; Mason, Sax A. (1980). "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 of the Chemical Society, Chemical Communications (7): 295. doi:10.1039/c39800000295.
  16. A. Dedieu (Editor) Transition Metal Hydrides 1991, Wiley-VCH, Weinheim. ISBN   0-471-18768-2


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