Hydride is the name given to the negative ion of hydrogen, H−. Practically, the term hydride has two distinct but overlapping meanings. In the chemical vernacular the term hydride refers to a hydrogen atom that formally reacts as a hydrogen anion under common conditions as well as hydrogen atoms directly bonded to metal atoms regardless of their reactivity. The second more antiquated meaning of hydride refers to any compounds hydrogen forms with another elements, ranging over most of the periodic table, groups 1–16. This second meaning is dealt with only in terms of formal nomenclature at the end of the article, the rest of the article concerns the popular meaning.
Hydrides' bonds range from very covalent to very ionic as well as multi-centered bonds and metallic bonding. Hydrides can be components of discrete molecules, oligomers or polymers, ionic solids, chemisorped monolayers, bulk metals, and other materials. While hydrides traditionally react as Lewis bases or reducing agents it is also common for some metal hydrides to react as hydrogen radicals and protons.
Various metal hydrides are currently being studied for use as a means of hydrogen storage in fuel cell-powered electric cars and in batteries. The group 14 hydrides are already of vast importance in storage battery technologies. They also have important uses in organic chemistry as powerful reducing agents, and many promising uses in a hydrogen economy.
See also: hydrogen anion.
Free hydride anions exist only under extreme conditions similar to the way that free protons also exist only under extreme conditions. Still there are many examples of hydrogen atoms that formally react as hydrides.
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 a powerful Lewis base. H− + H+ → H2; ΔH = −1676 kJ/mol
The low electron affinity of hydrogen and the strength of the H–H bond (∆HBE = 436 kJ/mol) means that the hydride ion would also be a strong reducing agent: H2 + 2e− ⇌ 2H−; E<s>o</s> = −2.25 V
Types of hydrides
According to the antiquated definition every element of the periodic table (except some noble gases) forms one or more hydrides. These compounds have been classified into three main types according to the nature of their bonding: Saline 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 or saline hydride, is a hydrogen atom bound to an extremely electropositive metal, generally an alkali metals or alkaline earth metals. In these materials the hydrogen atom is viewed as a pseudohalide. Saline hydrides are insoluble in conventional solvents, reflecting their nonmolecular structures. Most ionic hydrides exist as "binary" materials involving only two elements including hydrogen. Ionic hydrides are used as heterogeneous bases and reducing reagents in organic synthesis.
C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2
Typical solvents for such reactions are ethers. Water and other protic solvents cannot serve as a medium for pure 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.
NaH + H2O → H2 (gas) + 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. 4 LiH + AlCl3 → LiAlH4 + 3 LiCl
According to the antiquated definition of hydride covalent hydrides cover all other compounds containing hydrogen. The more contemporary definition limits hydrides to hydrogen atoms that formally react as hydrides and hydrogen atoms bound to metal centers. 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 import segment of covalent hydrides are complex metal hydrides, powerful soluble hydrides commonly used in synthetic procedures.
Molecular hydrides often involve additional ligands such as, 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 aluminum hydride and hindered reagents such as DIBAL.
Interstitial hydrides can exist as discrete molecules or metal clusters in which they are atomic centers in a defined multi-centered multi-electron bonds. Interstitial hydrides can also exist within bulk materials such as bulk metals or alloys at which point their bonding is generally considered metallic.
Many 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 is a consequence of interstitial hydrides.
A notable example of an interstitial binary hydrides is palladium which absorbs up to 900 times its own volume of hydrogen at room temperatures, forming palladium hydride, and has been considered as a means to carry hydrogen for vehicular fuel cells. 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.
Transition metal hydrido complexes
Transition metal hydride include what can be called covalent hydrides as well as interstial hydrides and other bridging hydrides.
Within the covalent transition metal hydrides there are two additional types of hydrides. The classical hydride also known as terminal hydrides involves a single bond between the hydrogen atom and a transition metal atom. Non-classical hydrides often referred to as dihydrogen complexes is when a dihydrogen molecule forms a sigma bonded complex with a metal center. There is a spectrum of bonding situation that span from a pure dihydrogen complex to a pure dihydride complex where the dihydrogen molecules bond has been fully broken.
There are many transition metal complexes that incorporate hydrides as ligands. Such compounds are often discussed in the context of organometallic chemistry. They are intermediates in many industrial processes that rely on metal catalysts, such as hydroformylation, hydrogenation, and hydrodesulfurization.
While all hydrogen atoms bound to metal center may be referred to as hydrides it does not necessarily have barring on their reactivity. The transition metal hydrides HCo(CO)4 and H2Fe(CO)4, are examples of acidic hydrides. Despite prediction made from fundamental elemental properties such as electronegativity the reactivity of metal complexes is complicated by their ligands chemistry.
The anion [ReH9]2− is a rare example of a molecular homoleptic metal hydride.
Isotopes of hydride
Protide, deuteride, and tritide are used to describe ions or compounds, which contain enriched hydrogen-1, deuterium or tritium, respectively.
Hydrides as a pseudohalide are capable of forming a unique form of bonding called dihydrogen bond. A dihydrogen bond exists between a negatively polarized hydride and a positively polarized hydrogen atom(hydron). This is similar to hydrogen bonding which exists between positively polarized protons and electronegative atoms with open lone pairs.
The second more antiquated meaning of hydride refers to any compounds hydrogen forms with other elements, ranging over groups 1–16. The following is a list of the nomenclature for the hydride derivatives of main group compounds according to this defintion: alkali and alkaline earth metals: metal hydride
boron: borane, BH3
gallium: gallane, GaH3
indium: indigane, InH3
thallium: thallane, TlH3
carbon: alkanes, alkenes, alkynes, and all hydrocarbons
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)
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
Examples: 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.
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).1
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