hydrides

Hydrides are chemical compounds formed when hydrogen is bonded to a more electropositive element. In ionic hydrides, hydrogen exists as the hydride ion (H−) within a lattice of metal cations. In covalent hydrides, hydrogen is bound to a nonmetal through covalent bonds, and in metallic (interstitial) hydrides hydrogen atoms occupy interstitial sites in a metal lattice. The term also encompasses complex and transition metal hydrides, where hydrogen can act as a ligand or dissolve in metal hosts.

Ionic hydrides are typically formed by alkali and alkaline earth metals, such as sodium hydride (NaH) and calcium hydride (CaH2). They are usually hard, high-melting solids that react vigorously with water and oxygen, releasing hydrogen gas and forming hydroxides or oxides.

Covalent hydrides involve hydrogen bonded to nonmetals. Common examples include ammonia (NH3), methane (CH4), borane (BH3), and silane (SiH4). These compounds span gases, liquids, and solids and exhibit a wide range of bonding environments from simple molecules to more complex frameworks.

Metallic hydrides consist of hydrogen dissolved in metal lattices, often forming interstitial compounds. They exhibit reversible hydrogen uptake and release and are of interest for hydrogen storage technologies. Examples include palladium hydride (PdHx) and titanium hydride (TiHx).

Complex hydrides, such as sodium borohydride (NaBH4) and lithium aluminium hydride (LiAlH4), contain hydrogen in a rigid, covalently bound anion framework and are important as reducing agents and in materials science.

Safety and reactivity vary widely: many hydrides react with water or air, sometimes releasing flammable hydrogen gas.