Hydride is the name given to the negative ion of hydrogen, H−. Although this ion does not exist except in extraordinary conditions, the term hydride is widely applied to describe compounds of hydrogen with other elements, particularly those of groups 1–16. The variety of compounds formed by hydrogen is vast, arguably greater than that of any other element. Every element of the periodic table (except some noble gases) forms one or more hydrides. These may be classified into three main types by the predominant nature of their bonding:
- Saline hydrides, which have significant ionic character,
- Covalent hydrides, which include the hydrocarbons and many other compounds, and
- Interstitial hydrides, which may be described as having metallic bonding.
- See also: hydrogen anion.
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, thus hydride is so basic that it is unknown in solution. The reactivity of the hypothetic hydride ion is dominated by its exothermic protonation to give dihydrogen:
- H− + H+ → H2; ΔH = −1676 kJ/mol
As a result, the hydride ion is one of the strongest bases known. It would extract protons from almost any hydrogen-containing species. The low electron affinity of hydrogen and the strength of the H–H bond (436 kJ/mol) means that the hydride ion would also be a strong reducing agent:
- H2 + 2e− ⇌ 2H−; E
o= −2.25 V
- H2 + 2e− ⇌ 2H−; E
In ionic hydrides the hydrogen behaves as a halogen and obtains an electron from the metal to form a hydride ion (H−), thereby attaining the stable electron configuration of helium by filling its 1s-orbital. The other element is a metal more electropositive than hydrogen, usually one of the alkali metals or alkaline earth metals. The hydrides are called binary if they only involve two elements including hydrogen. Chemical formulae for binary ionic hydrides typically MH (as in LiH). As the charge on the metal increases, the M-H bonding becomes more covalent as in MgH2 and AlH3. Ionic hydrides are commonly encountered as basic reagents in organic synthesis:
Such reactions are heterogeneous, the KH does not dissolve. Typical solvents for such reactions are ethers. Water cannot serve as a medium for pure ionic hydrides or LAH because the hydride ion is a stronger base than hydroxide. Hydrogen gas is liberated in a typical acid-base reaction.
- 4 LiH + AlCl3 → LiAlH4 + 3 LiCl
In covalent hydrides, hydrogen is covalently bonded to more electropositive element such as p-block (boron, aluminium, and Group 4-7) elements as well as beryllium. Common compounds include the hydrocarbons and ammonia could be considered as hydrides of carbon and nitrogen, respectively. Charge neutral covalent hydrides that are molecular are often volatile at room temperature and atmospheric pressure. Some covalent hydrides are not volatile because they are polymeric—i.e. nonmolecular—such as the binary hydrides of aluminium and beryllium. Replacing some hydrogen atoms in such compounds with larger ligands, one obtains molecular derivatives. For example, diisobutylaluminium hydride (DIBAL) consists of two aluminium 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 aluminum hydride and hindered reagents such as DIBAL.
Transition metal hydrido complexes
Most transition metal complexes form molecular compounds described as hydrides. Usually such compounds are discussed in the context of organometallic chemistry. Transition metal hydrides are intermediates in many industrial processes that rely on metal catalysts, such as hydroformylation, hydrogenation, and hydrodesulfurization. Two famous examples, HCo(CO)4 and H2Fe(CO)4, are acidic thus demonstrating that the term hydride is used very broadly. Deprotonation of dihydrogen complexes gives metal hydrides. The anion [ReH9]2- is a rare example of a molecular homoleptic metal hydride.
Interstitial hydrides of the transitional metals
Structurally related to the saline hydrides, the transition metals form binary hydrides which are often non-stoichiometric, with variable amounts of hydrogen atoms in the lattice, where they can migrate through it. In materials engineering, the phenomenon of hydrogen embrittlement is a consequence of interstitial hydrides. Palladium absorbs up to 900 times its own volume of hydrogen at room temperatures, forming palladium hydride, and was therefore once thought as a means to carry hydrogen for vehicular fuel cells. Hydrogen gas is liberated proportional to the applied temperature and pressure but not to the chemical composition.
Interstitial hydrides show certain promise as a way for safe hydrogen storage. During last 25 years many interstitial hydrides were 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, which is not enough for automotive applications.
Various metal hydrides are currently being studied for use as a means of hydrogen storage in fuel cell-powered electric cars and batteries. They also have important uses in organic chemistry as powerful reducing agents, and many promising uses in hydrogen economy. The following is a list of main group hydride nomenclature:
- alkali and alkaline earth metals: metal hydride
- boron: borane and rest of the group as metal hydride
- carbon: alkanes, alkenes, alkynes, and all hydrocarbons
- silicon: silane
- germanium: germane
- tin: stannane
- lead: plumbane
- nitrogen: ammonia ('azane' when substituted), hydrazine
- phosphorus: phosphine ('phosphane' when substituted)
- arsenic: arsine ('arsane' when substituted)
- antimony: stibine ('stibane' when substituted)
- bismuth: bismuthine ('bismuthane' when substituted)
According to the convention above, the following are "hydrogen compounds" and not "hydrides":
- oxygen: water ('oxidane' when substituted), hydrogen peroxide
- sulfur: hydrogen sulfide ('sulfane' when substituted)
- selenium: hydrogen selenide ('selane' when substituted)
- tellurium: hydrogen telluride ('tellane' when substituted)
- halogens: hydrogen halides
- nickel hydride: used in 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
- arsine: used for doping 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, fertilizer, many other industrial uses
- hydrogen sulfide: component of natural gas, important source of sulfur
- Chemically, even water and hydrocarbons could be considered hydrides.
Isotopes of hydride
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).