A hydride is the anion of hydrogen, H‑, or more generally a compound in which one or more hydrogen centres exhibit nucleophilic, reducing or basic characteristics. In compounds regarded as hydrides, the hydrogen atom is bonded to a more electropositive element or group. The polarity of the basic metal–hydrogen bond causes hydrides to react vigorously with water, frequently in an irreversible manner.
Hydride Categories
There are three primary types of hydrides: (i) salt or ionic hydrides, (ii) metallic hydrides and (iii) covalent hydrides, which are distinguished by the nature of the chemical bond. A fourth variety, the dimeric hydride (e.g. borane, BH3), may also be differentiated by its structure.
Salt or Ionic Hydrides are characterised by the presence of hydrogen in the form of a negatively charged ion (i.e. H‑). Generally, the hydrides of alkali and alkaline earth metals are classified as salt hydrides (with the possible exception of beryllium hydride, BeH2, and magnesium hydride, MgH2). These metals react directly with hydrogen at high temperatures (30–700 °C [570–1300 °F]) to form hydrides with general formulas MH and MH2. In their pure state, these compounds appear as white crystalline solids, although they are often grey because of minor metal impurities.
Metallic Hydrides are produced by heating hydrogen gas with metals or their alloys. Researchers have studied the compounds of electropositive transition metals extensively, particularly those in the scandium, titanium and vanadium families. For example, in the titanium family titanium (Ti), zirconium (Zr) and hafnium (Hf) form non‑stoichiometric hydrides when they absorb hydrogen and release heat. These hydrides display chemical reactivity comparable to that of finely divided metal. They remain stable in air at ambient temperature, yet they react when heated in air or with acidic compounds. They exhibit the appearance of the metal and occur as grey‑black solids. The metal generally exists in a +3 oxidation state, and the bonds are predominantly ionic.
Covalent Hydrides are primarily compounds of hydrogen and non‑metals. Their bonds consist of shared electron pairs between atoms with comparable electronegativity. Consequently, most non‑metal hydrides are volatile compounds held together in the condensed phase by relatively weak intermolecular Van der Waals interactions. Covalent hydrides usually exist as liquids or gases with low melting and boiling points, except in cases (such as water) where hydrogen bonding alters their properties. Covalent hydrides may be formed from boron (B), aluminium (Al) and gallium (Ga) of group 13 of the periodic table. Ionic hydrogen species of boron (BH4‑) and aluminium (AlH4‑) are widely utilised as hydride sources.
Uses of Hydrides
* Hydrides such as sodium borohydride, lithium aluminium hydride, diisobutyl aluminium hydride (DIBAL) and Superhydride are frequently employed as reducing agents in chemical synthesis. The hydride adds to an electrophilic centre, usually unsaturated carbon.
* Hydrides such as sodium hydride and potassium hydride are used as strong bases in organic synthesis. The hydride reacts with a weak Brønsted acid and liberates H2.
* Hydrides such as calcium hydride are used as drying agents to remove traces of water from organic solvents. The hydride reacts with water to form hydrogen and a hydroxide salt. Consequently, the dry solvent may be distilled or removed under vacuum from the solvent pot.
* Hydrides are important in battery technologies such as the nickel‑metal hydride battery. Various metallic hydrides have been investigated for their suitability as hydrogen storage media for fuel cell‑powered electric vehicles and other aspects of a hydrogen economy.
* Hydride complexes serve as catalysts and catalytic intermediates in numerous homogeneous and heterogeneous catalytic cycles. Examples include catalysts for hydrogenation, hydroformylation, hydrosilylation and hydrodesulphurisation. Certain enzymes, such as hydrogenase, operate via hydride intermediates. The energy carrier nicotinamide adenine dinucleotide acts 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
Converters & Calculators
