lithium aluminum hydride

Hydride

[hahy-drahyd, -drid]
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.

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 hydrogen economy.

Every element of the periodic table (except some noble gases) forms one or more hydrides. These compounds 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.

Hydride ion

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. This however is deceptive since the proton is so acidic it is also 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 (∆HBE = 436 kJ/mol) means that the hydride ion would also be a strong reducing agent:
H2 + 2e ⇌ 2H; Eo = −2.25 V

Ionic hydrides

In ionic, or saline, hydrides, the hydrogen is viewed as a pseudohalide. The saline hydrides are insoluble in conventional solvents, reflecting their nonmolecular structures. H has stable electron configuration of helium with a filled 1s-orbital. Ionic hydrides also feature an electropositive metal, usually one of the alkali metals or alkaline earth metals. These 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:
C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2
Such reactions are heterogeneous because 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.
NaH + H2O → H2 (gas) + NaOH ΔH = −83.6 kJ/mol, ΔG = −109.0 kJ/mol

Alkali metal hydrides react with metal halides. Lithium aluminium hydride (often abbreviated as LAH) arises from reactions with aluminium chloride.

4 LiH + AlCl3 → LiAlH4 + 3 LiCl

Covalent hydrides

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 that contain one or more hydride ligands. Usually such compounds are 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.

Deprotonation of dihydrogen complexes gives metal hydrides.

Two famous examples of transition metal hydrides are HCo(CO)4 and H2Fe(CO)4, are acidic thus demonstrating that the term hydride is used very broadly. The anion [[Potassium nonahydridorhenate|[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.

Nomenclature

The following is a list of the nomenclature for the hydride derivatives of main group compounds:

According to the convention above, the following are "hydrogen compounds" and not "hydrides":

Examples:

Isotopes of hydride

Protide, deuteride, and tritide are used to describe ions or compounds, which contain enriched hydrogen-1, deuterium or tritium, respectively.

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).

See also

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