The binary compounds of hydrogen with transition metals are called metallic hydrides or interstitial hydrides. Some of the transition metals react vigorously with hydrogen but others do not. A few transition metals can only absorb or occlude hydrogen without undergoing any chemical reaction. The reaction of metals Ti, Zr, Hf and lanthanides with hydrogen is exothermic and can be formed easily. But Mo, W, Mn, Fe, Co, Ru, Pt, etc., react with hydrogen endothermically indicating their slow reactivity. Therefore, the hydrides of these metals are either unstable or unknown.
- The hydrides are formed by the absorption of hydrogen within the interstices or holes present in the metallic structure. The stoichiometry is not in accordance with the laws of chemical combinations.
- The hydrides which are obtained by physical adsorption of hydrogen at a definite position within the crystal lattice. The properties of hydrides formed remain those of metals.
- The metal hydrides formed by reactive metals have crystal structure quite different from the metals. The hydrides, in such cases, appear definite to be loose chemical compounds. Even in these metallic hydrides, the chemical composition is variable.
- By the direct combination of metal and hydrogen: The transition metals when heated in the presence of hydrogen atmosphere from metallic hydrides.
- From halides: The hydrides of nickel, cobalt, iron and chromium are obtained by passing hydrogen gas through the suspension of anhydrous metal chlorides in organic solvents in presence of Grignard reagents.
- By the action of LiAIH4: The hydrides of zinc and cadmium etc,. are prepared by treating their dimethyl with LiAlH4.
Some of these hydrides are used as catalysts from a certain organic reaction. Thus, titanium hydride can be used as a catalyst for hydrogenation of olefins, reduction of nitrobenzene to aniline, and nitriles to amines.