HYDRIDE

a compound of hydrogen with another element. Three basic types of hydrides may be distinguished in a classification based on the type of chemical bond involved: 1. Saline, or ionic: hydrides in which hydrogen is present as a negatively charged ion, H-. In these compounds, hydrogen displays a family resemblance to the halogens, such as fluorine and chlorine. Saline hydrides react vigorously with water, giving off large volumes of gaseous hydrogen; this property renders them useful as light, portable sources of hydrogen. Examples of binary saline hydrides are sodium hydride, NaH, and calcium hydride, CaH2. Examples of complex saline hydrides are 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). 2. Metallic (formerly termed interstitial): alloylike hydrides that 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 sometimes harder than the metals from which they are made. Such compounds are regarded as intermediate in nature between salts and alloys. Metallic hydrides have been regarded as made up 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. Examples are titanium hydride, TiH2, and thorium dihydride, ThH2. 3. Covalent: hydrides that are primarily compounds of hydrogen and nonmetals. In such hydrides, the bonds are evidently electron pairs shared by atoms of comparable electronegativities. Water (H2O), ammonia (NH3), and hydrogen sulfide (H2S) are covalent hydrides, as are methane (CH4) and the other hydrocarbons. Other examples of covalent hydrides include silane, SiH4; arsine, AsH3; germane, GeH4; aluminum borohydride, Al(BH4)3; and digermane, Ge2H6. Covalent hydrides are liquids or gases that have low melting and boiling points, except in those cases (such as water) where their properties are modified by hydrogen bonding. A fourth type of hydride may also be identified based on structure. Dimeric or polymeric hydrides are those in which hydrogen is presumed to have formed a connecting bridge between metal or metalloid atoms. Classic examples are furnished by the numerous hydrides of borone.g., diborane, B2H6; pentaborane, B5H9; and decaborane, B10H14. When such hydrides burn, they give off considerably more energy than is provided by carbonaceous fuels and are of interest as high-energy fuels for rockets. 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.