Hydride Types, the Hydride Ion, and Occlusion

Saline (ionic) hydrides, like NaH and CaH2, are stoichiometric compounds with a fixed, well-defined formula, while metallic (interstitial) hydrides, formed by transition metals like Pd or Ti absorbing hydrogen into their lattice, are typically non-stoichiometric (a variable H-to-metal ratio, like PdH0.6) — this statement is correct. Group 13 hydrides (like B2H6) are electron-deficient (too few electrons for a normal 2-centre-2-electron bond at every position, requiring 3-centre-2-electron bridging bonds instead), while Group 15 hydrides (like NH3) are electron-rich (they have a lone pair beyond what is needed for the bonds present) — also correct. But claiming BeH2 is monomeric while MgH2 is polymeric gets this backwards: BeH2 itself typically adopts a polymeric, bridged chain structure (similar in spirit to how B2H6 bridges), not a simple monomeric form. And NaH does react with water to liberate H2 (true, a classic ionic-hydride hydrolysis), but B2H6 also reacts vigorously with water (hydrolysing to boric acid and H2), so claiming B2H6 does NOT react with water is also false — putting these two false claims together correctly identifies which statements do not hold.

H- (hydride ion) is a stronger base than OH- (hydroxide ion). When an ionic hydride like NaH is added to water, the hydride ion deprotonates a water molecule (acting as the stronger base, pulling a proton away), giving hydroxide ion and hydrogen gas: H-(aq) + H2O(l) → OH-(aq) + H2(g). This is not an acid-base reaction going the other way (hydride does not get protonated to form H3O+ style species in any sense relevant here), and it is very much a real, vigorous reaction, not a non-reaction — this is exactly why ionic hydrides are handled with care around any trace of moisture.

Across the alkali metal hydrides, ionic character increases going down the group (LiH < NaH < KH < RbH < CsH), for the same reason ionic character increases in any alkali metal salt series — larger, more electropositive cations hold their bonding electrons less tightly, pushing the bond further toward fully ionic. An electron-precise hydride is one where the number of available electrons exactly matches what is needed for a normal 2-centre-2-electron bond at every position, with nothing left over and nothing missing — CH4 (Group 14) is the classic electron-precise example, distinct from electron-deficient B2H6 (Group 13) and electron-rich NH3/H2O (Groups 15/16, each carrying extra lone pairs). Separately, calcium reacting directly with dihydrogen gas forms calcium hydride, CaH2, historically also called hydrolith — a trade name worth recognising specifically for CaH2, since it is used as a portable, compact source of hydrogen gas (reacting readily with water to release H2 on demand).

When certain metals (like palladium, platinum, or titanium) take up hydrogen gas not just on their surface but absorbed throughout the bulk of the metal lattice, this process is called OCCLUSION — distinct from simple surface adsorption (which stays at the surface) and distinct from hydrogenation (a chemical reaction adding H2 across a double/triple bond in an organic molecule) or dehydrogenation (removing H2 from a molecule). Occlusion is exactly the process behind metallic/interstitial hydride formation covered above — the absorbed hydrogen occupies interstitial sites within the metal lattice, which is also why the resulting hydride is typically non-stoichiometric, since the amount absorbed can vary with pressure and temperature rather than following one fixed ratio.