metalhydrogen

Metalhydrogen is an ambiguous term that can refer to two related, but distinct, concepts in chemistry and physics. In many contexts, it denotes metal hydrides, binary compounds formed between hydrogen and metals. In other contexts, it refers to metallic hydrogen, a hypothesized state of hydrogen that occurs under extreme pressure.

Metal hydrides are formed when hydrogen is absorbed by metals or alloys, occupying sites in the metal lattice. They can be broadly categorized as ionic hydrides, where hydrogen behaves as H−, covalent hydrides with more shared bonding, or metallic hydrides in which hydrogen participates in a delocalized metallic bonding network within the lattice. Interstitial hydrides are common, with hydrogen atoms occupying interstitial sites in the metal’s crystal structure and often enabling reversible storage of hydrogen.

Notable examples include sodium hydride (NaH), magnesium hydride (MgH2), titanium hydride (TiH2), and palladium hydride (PdH_x). Properties vary widely: some hydrides have high hydrogen storage capacities by weight, while others release hydrogen only at elevated temperatures. They are of interest for chemical synthesis, hydrogen storage, and catalytic applications, and some serve as reagents in organic and inorganic chemistry.

Applications of metal hydrides include hydrogen storage for energy systems, as components in hydrogenation reactions, and as electrodes in metal hydride batteries. Nickel–metal hydride (NiMH) batteries, for example, use metal hydride alloys as negative electrodes, enabling reversible hydrogen absorption and desorption during charging and discharging.

Metallic hydrogen, a separate concept, refers to hydrogen in a metallic state anticipated at extremely high pressures. It remains an area of active research and debate, with experimental claims and theoretical work continuing to explore its existence and properties at high pressure and potentially high temperature.