hydride

hydride, any of a class of chemical compounds in which hydrogen is combined with another element. Three basic types of hydrides—saline (ionic), metallic, and covalent—may be distinguished on the basis of type of chemical bond involved. A fourth type of hydride, dimeric (polymeric) hydride, may also be identified on the basis of structure (see borane). Aluminum and, possibly, copper and beryllium hydrides are nonconductors that exist in solid, liquid, or gaseous forms. All are thermally unstable, and some explode on contact with air or moisture.

Saline (ionic) hydrides

Saline, or ionic, hydrides are defined by the presence of hydrogen as a negatively charged ion, H⁻. The saline hydrides are generally considered those of the alkali metals and the alkaline-earth metals (with the possible exception of beryllium hydride, BeH₂, and magnesium hydride, MgH₂). These metals enter into a direct reaction with hydrogen at elevated temperatures (300–700 °C [570–1,300 °F]) to produce hydrides of the general formulas MH and MH₂. Such compounds are white crystalline solids when pure but are usually gray, owing to trace impurities of the metal. Structural studies show that these compounds contain a hydride anion, H⁻, with a crystallographic radius that is dependent on the identity of the metal but intermediate to that of the fluoride ion, F⁻ (1.33 angstroms), and the chloride ion, Cl⁻ (1.84 angstroms). This radius is somewhat smaller than the calculated radius for the free H⁻ ion of 2.08 angstroms. This value has not been observed experimentally, which probably can be attributed to two factors: (1) the electron cloud of H⁻ is diffuse and easily compressible, and (2) there is likely some covalent character to the metal-hydrogen bond. The hydride ion in the saline hydrides is a strong base, and these hydrides react instantly and quantitatively with the hydrogen ion (H⁺) from water to produce hydrogen gas and the hydroxide ion in solution. H⁻ + H₂O → H₂ + OH⁻ Because saline hydrides react vigorously with water, giving off large volumes of gaseous hydrogen, this property renders them useful as light, portable sources of hydrogen.

The alkaline-earth metals beryllium and magnesium also form stoichiometric MH₂ hydrides. However, these hydrides are more covalent in nature. It is difficult to isolate pure BeH₂, but its structure is thought to be polymeric with bridging hydrogen atoms. Other examples of binary saline hydrides include sodium hydride, NaH, and calcium hydride, CaH₂. Examples of complex saline hydrides include lithium aluminum hydride, LiAlH₄, and sodium borohydride, NaBH₄, both of which are commercial chemicals used as reducing agents (substances that provide electrons in oxidation-reduction reactions).

Metallic hydrides

The transition metals and inner transition metals form a large variety of compounds with hydrogen, ranging from stoichiometric compounds to extremely complicated nonstoichiometric systems. (Stoichiometric compounds have a definite composition, whereas nonstoichiometric compounds have a variable composition.) Metallic (formerly termed interstitial) alloylike hydrides possess some of the characteristics of metals, such as lustre and strong electrical conductivity. They tend, however, to have variable physical properties, with some being more brittle and others being harder than the metals from which they are made. Such compounds are regarded as intermediate in nature between salts and alloys. Metallic hydrides essentially consist of protons (positive hydrogen ions, H⁺) and metal atoms in an electron sea. The lustre and electrical conductivity are attributed to the relative freedom of electron movement in the hydride.

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Metallic hydrides are formed by heating hydrogen gas with the metals or their alloys. The most thoroughly studied compounds are those of the most electropositive transition metals (the scandium, titanium, and vanadium families). For example, in the titanium family, titanium (Ti), zirconium (Zr), and hafnium (Hf) form nonstoichiometric hydrides when they absorb hydrogen and release heat. These hydrides have a chemical reactivity similar to the finely divided metal itself, being stable in air at ambient temperature but reactive when heated in air or with acidic compounds. They also have the appearance of the metal, being grayish black solids. The metal appears to be in a +3 oxidation state, and the bonding is predominantly ionic. These hydrides are used as reducing agents in some processes (e.g., metallurgy). The inner transition metals (the lanthanoids and actinoids) also form nonstoichiometric hydrides. For example, lanthanum (La) reacts with hydrogen gas at one atmosphere pressure with little or no heating to produce a black solid that inflames in air and reacts vigorously with water. Uranium hydride (UH₃) is the most important hydride of the actinoid metals. This pyrophoric black powder is prepared by reaction with hydrogen at 300 °C (570 °F). 2U + 3H₂ → 2UH₃ This compound is useful chemically for the preparation of uranium compounds.