Hydrides

compounds of hydrogen with other elements. Depending on the nature of the hydrogen bond, one can differentiate three types of hydrides: ionic, metallic, and cova-lent.

Hydrides of alkali and alkaline earth metals are ionic (saltlike) hydrides. They are white crystalline substances, stable under ordinary conditions, and only upon heating do they decompose, without melting, into metal and hydrogen (except LiH, which melts at 680° C). They are vigorously decomposed by water, with the evolution of hydrogen. They are made by the reaction of hydrogen with metals at 200°-600° C. LiH and NaH are used in organic synthesis as reducing and condensing agents; CaH₂ is used for drying and to detect water in organic solvents, for preparing powdered metals from oxides, and for making hydrogen. A solution of NaH in a molten alkali is used to remove the scale from metal articles. The double hydrides—the borohydrides, MeBh₄, and aluminum hydrides, MeAlH₄—also have an ionic structure. They are extensively used in organic synthesis as effective reducing agents.

Hydrides of the transition metals are of the metallic type, since they are similar to metals because of the nature of their chemical bond. For the most part these hydrides are compounds of variables, and the formulas below give only their limiting hydrogen content. Many metals can absorb a considerable amount of hydrogen, giving solid solutions that retain the crystalline structure of the particular metal. On the other hand, the structures of true hydrides differ from those of the initial metals. The formation of two types of hydrides, MeH₂ and MeH₃, is characteristic of metals of Group III of the periodic system (the Sc subgroup and the lanthanides). Group IV metals (the Ti subgroup) give MeH₂ hydrides, and Group V metals (the vanadium subgroup) give MeH hydrides. The metal hydrides of those groups are brittle gray or black solids and are produced by the action of hydrogen on the finely divided metals at high temperatures. Metals of Groups VI, VII, and VIII, except for palladium, do not form definite chemical compounds when they absorb hydrogen.

Transition metal hydrides can be used to catalyze various chemical reactions. The ability of metals to form hydrides is used in high-vacuum technology to bind hydrogen. The quality of the metals is impaired by hydride formation—for example, when water vapor acts on incandescent metal, or in the electrolytic isolation of metals (so-called hydrogen em-brittlement).

The hydrides of transition metals of Groups I and II of the periodic system, as well as of Group III (the Al subgroup) are not formed by reaction of metal with hydrogen. They are made, for example, by reducing compounds of the metals with lithium aluminum hydride, LiAlH₄, in an ether solution. Upon heating they all readily decompose into metal and hydrogen.

Covalent hydrides are formed by nonmetals of Groups IV, V, VI, and VII of the periodic system, as well as by boron. Apart from the simplest compounds of this type (methane, CH₄; silane, SiH₄; and so on), which are gases, hydrides with a large number of atoms of an element joined together to form chains are known—for example, the silanes, Si„H_2n+2. The simplest boron hydride, BH₃, does not exist, and the boron hydrides have a complex structure. Hydrides of elements of the first periods are very stable; those of heavy elements are extremely unstable. Many hydrides (B₂H₆, SiH₄, and PH₃) ignite readily in air. B₂H₆ and SiH₄ are decomposed by water, with the liberation of hydrogen. The hydrides of elements in Groups V, VI, and VII are not decomposed by water. Numerous covalent hydride derivatives are known, in which some of the hydrogen atoms are replaced by halogen or metal atoms, as well as by alkyl and other groups. Covalent hydrides are prepared by the direct reaction of the elements, by decomposing metallic compounds with water or acids, or by reducing halides and other compounds with hydrides, borohydrides, and aluminum hydrides of alkali metals. Thermal decomposition of hydrides is a method of preparing very pure elements (for example, silicon and germanium).