Hydrides of d-block (Group No. 3, 4, 5 and 6) and f-block are metallicor interstitial hydrides because in these hydrides, hydrogen occupies some interstitial sites in the metal lattice producing distortion. These hydrides are also called non-stoichiometric hydrides as, the law of constant composition does not hold good for them, e.g., LaH2.87,YbH2.55 , TiH1.5–1.8,ZrH1.3–1.75,VH0.56, NiH0.6–0.7,PdH0.6–0.8 etc. Alkali metals do not form these types of hydrides because alkali metals donate lone pair of electrons to hydrogen which forms H– ion. As, H– ion is formed by complete transfer of electrons so, alkali metal and H atom are in fixed stoichiometric ratio.