Hydride

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A hydride is a compound of hydrogen with more electropositive elements.

Originally, the term hydride was reserved strictly for compounds containing hydride ions, usually in combination with metals, but the definition has been broadened to compounds (usually simple binary) involving hydrogen in direct bond with another element.

Hydrides can be roughly classified into three main types by the nature of bonding and structure:

  • Ionic hydrides
  • Covalent hydrides
  • Transitional metal hydrides, interstitial hydrides.

In main group element hydrides electronegativity of an element respective to hydrogen determines the compound to be either of the first two types. An electropositive metal, from the left side of the Periodic table, forms ionic hydrides whereas an electronegative element (usually from the table's right side) forms covalent hydrides; however silane is one of the exceptions.

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[edit] 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).

[edit] Ionic hydrides

In ionic hydrides the hydrogen behaves as a halogen and obtains an electron from the metal to form a hydride ion (H−), thereby obtaining the stable electron configuration of helium or filling up the s-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 are either MH (as in LiH) or MH2 (as in MgH2). Gallium, indium, thallium and lanthanide hydrides are also ionic.

Their structures are purely crystalline.

They are prepared by reacting the element with hydrogen gas, under pressure if needed.

Ionic hydrides are usually used as reducing agents in synthetic chemistry, but they are too strongly basic and reactive to be used in pure form. Hydrides of lesser reactivity are more commonly used especially if the reaction can be carried out in water or organic solvents. Reduction by sodium borohydride (NaBH4) can be carried out in water. If a reactive hydride has to be used, the reduction will be carried out in a medium that readily dissolves the hydride ion without decomposition, for instance in liquid ammonia. Pure binary hydrides are still not often used even in those criteria. Lithium hydride is reduced in reactivity by forming lithium aluminium hydride (often abbreviated as LAH) with aluminium chloride.

4 LiH + AlCl3 → LiAlH4 + 3 LiCl

Water itself 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 if the hydride is immersed. The liberation is a typical acid-base reaction.

H− + H2O → H2 (gas) + OH−

[edit] Covalent hydrides

As the name suggests, the hydrogen is covalently bonded to more electropositive p-block (boron, aluminium and Group 4-7) elements and beryllium. Hydrogen does not form any true ionic compunds as a metal under normal conditions. Common compounds including hydrocarbons, ammonia and hydrazine could be considered as hydrides of carbon and nitrogen but the term is only used for collectively naming all hydrogen compounds of an element. Ammonia is never called nitrogen trihydride. The hydride nomenclature does not suffice to provide a unique name for each hydrocarbon. Choice of nomenclature, either as metal hydrides or in parallel to alkane, alkene and alkyne, mostly depends on the perspective of the scientist.

Covalent hydrides behave as molecules with the weak London forces and hence are volatile at room temperature and atmospheric pressure. Aluminium and beryllium hydrides are polymeric because of three center bond.

Properties of covalent hydrides vary individually.

The following is a list of main group hydride nomenclature:

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

[edit] Interstitial hydrides of the transitional metals

Their bonding nature vastly differs from element to element and changes according to external criteria such as temperature, pressure and electric current. Titanium and coinage metal hydrides are polymeric.

Other transitional metal hydrides are interstitial in nature. In these, molecules of hydrogen dissociate and hydrogen atoms settle in the octahedral or tetrahedral holes in the metal crystal lattice called the interstitial sites. Interstitial hydrides often have non-stoichiometric nature. Hydrogen atoms trapped in the lattice can migrate through it, reacting with impurities and worsening the properties of the material. In materials engineering this is known as hydrogen embrittlement.

For instance, Palladium hydride is not yet clearly considered a compound though it possibly forms Pd2H. The dihydrogen unit (H2) may also appear in the palladium lattice. Palladium adsorbs up to 900 times its own volume of hydrogen at room temperatures and was therefore once thought as a means to carry hydrogen for vehicle 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 percents of hydrogen, which is not enough for automotive applications.

[edit] Usage

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.

Examples:

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