Write a note on metallic or non-stoichiometric hydrides.

(N/A) Metallic hydrides are formed by many $d$-block and $f$-block elements.
However,the metals of group $7$,$8$,and $9$ do not form hydrides. Even from group $6$,only chromium forms $CrH$. These hydrides conduct heat and electricity,though not as efficiently as their parent metals.
Unlike saline hydrides,they are almost always non-stoichiometric,being deficient in hydrogen.
For example: $LaH_{2.87}$,$YbH_{2.55}$,$TiH_{1.5-1.8}$,$ZrH_{1.3-1.75}$,$VH_{0.56}$,$NiH_{0.6-0.7}$,$PdH_{0.6-0.8}$,etc. In such hydrides,the law of constant composition does not hold.
In these hydrides,hydrogen occupies interstitial space in the metal lattice,producing distortion without any change in its type. Consequently,they are termed as interstitial hydrides.
Except for the hydrides of $Ni$,$Pd$,$Ce$,and $Ac$,other hydrides of this class have a lattice different from that of the parent metal.
The property of absorption of hydrogen on transition metals is widely used in catalytic reduction/hydrogenation reactions for the preparation of a large number of compounds. Some metals (e.g.,$Pd$,$Pt$) can accommodate a very large volume of hydrogen and,therefore,can be used as storage media.
This property has high potential for hydrogen storage and as a source of energy.