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why are ionic hydrides known as saline 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.
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