metallic (interstitial) hydride

Picture stacking oranges in a crate: between the spheres there are always small empty gaps. A block of transition metal is just such a packing of metal atoms, and into those tiny gaps — the interstices — hydrogen atoms can slip without rebuilding the lattice. Many transition metals soak up hydrogen this way like a sponge soaks up water, forming metallic or interstitial hydrides. The metal keeps its shiny, conducting character; the hydrogen simply lodges in the holes.

Because the hydrogen fills whatever holes it can reach rather than satisfying a fixed valence, these hydrides are usually non-stoichiometric: their formulas come out as awkward fractions like PdH0.6 or TiH1.7, varying with temperature and pressure. The hydrogen is thought to sit largely as atoms or even partly ionized, donating electrons into the metal's conduction band, which is why these solids stay electrically conducting and metallic-looking. A famous trick: palladium can absorb hundreds of times its own volume of hydrogen gas at room temperature, then release it on heating, acting like a chemical sponge for the gas.

These hydrides matter for hydrogen storage and for catalysis. Metal hydride alloys store hydrogen safely and reversibly for fuel cells and batteries (the nickel-metal-hydride cell is named for exactly this), and dissolved hydrogen on metal surfaces is the active species in countless hydrogenation reactions. The honest caveat: the very same dissolved hydrogen can wreck a metal — hydrogen embrittlement, where absorbed H atoms make steel crack — so the sponge-like uptake is a double-edged property, useful in storage but dangerous in structural metals.