A hydride is an anion of hydrogen, H−, or more commonly, it is a compound in which one or more hydrogen centers have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, the hydrogen atom is bonded to a more electropositive element or group. However, the basic metal‑hydrogen bond polarity causes hydrides to react vigorously with water, often in an irreversible manner.
Hydride Categories
There are three basic types of hydrides (i) saline hydride or ionic hydride, (ii) metallic hydride, and (iii) covalent hydride which may be distinguished on the basis of type of chemical bond involved. A fourth type of hydride, the dimeric hydride (of which borane, BH3, is an example), may also be identified on the basis of structure.
Saline, or ionic, hydrides are defined by the presence of hydrogen as a negatively charged ion (i.e., H−). The saline hydrides are generally considered to be the hydrides of the alkali metals and the alkaline earth metals (ith the possible exception of beryllium hydride, BeH2, and magnesium hydride, MgH2). These metals enter into a direct reaction with hydrogen at elevated temperatures (30–700 °C [570–1300 °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.
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, and the bonding is predominantly ionic.
Covalent hydrides are primarily compounds of hydrogen and nonmetals, in which the bonds are evidently electron pairs shared by atoms of comparable electronegativities. For example, most nonmetal hydrides are volatile compounds, held together in the condensed state by relatively weak van der Waals intermolecular interactions. Covalent hydrides are liquids or gases that have a low melting point and allow boiling point, except in those cases (such as water) where their properties are modified by hydrogen bonding. Covalent hydrides can be formed from boron (B), aluminum (Al), and gallium (Ga) of group 13 in the Periodic Table (Table 3.7). Ionic hydrogen species of both boron (BH4−) and aluminum (AlH4−) are extensively used as hydride sources.
Hydride Application
* Hydrides such as sodium borohydride, lithium aluminium hydride, diisobutylaluminium hydride (DIBAL) and super hydride, are commonly used as reducing agents in chemical synthesis. The hydride adds to an electrophilic center, typically unsaturated carbon.
* Hydrides such as sodium hydride and potassium hydride are used as strong bases in organic synthesis. The hydride reacts with the weak Bronsted acid releasing H2.
* Hydrides such as calcium hydride are used as desiccants, i.e. drying agents, to remove trace water from organic solvents. The hydride reacts with water forming hydrogen and hydroxide salt. The dry solvent can then be distilled or vacuum transferred from the "solvent pot".
* Hydrides are important in storage battery technologies such as nickel-metal hydride battery. Various metal hydrides have been examined for use as a means of hydrogen storage for fuel cell-powered electric cars and other purposed aspects of a hydrogen economy.
* Hydride complexes are catalysts and catalytic intermediates in a variety of homogeneous and heterogeneous catalytic cycles. Important examples include hydrogenation, hydroformylation, hydrosilylation, hydrodesulfurization catalysts. Even certain enzymes, the hydrogenase, operate via hydride intermediates. The energy carrier nicotinamide adenine dinucleotide reacts as a hydride donor or hydride equivalent.
Reference
James G.Speight: Natural Water Remediation-Chemistry and Technology. 2020, Pages 91-129
https://en.wikipedia.org/wiki/Hydride
