In which of the following hydride law of constant composition does not hold good.

Interstitial hydrides [Transition metal hydrides] are non-stoichiometric hydrides. They do not follow the valency rules. The formula of non stoichiometric hydrides does not correspond to any of the oxidation state of metal.
Zr H1.3 - 1.75 ⇒ Non - stoichiometric hydrides
Zr H⇒ [Kr] 4d2 5s2 ⇒ Zr⁺ H- ⇒ ZrH
                               Zr⁺² +2H- ⇒ ZrH₂
                               Zr⁺³ + 3H- ⇒ ZrH₃
                               Zr⁺⁴ + 4H- ⇒ ZrH₄
The formula of Zirconium hydride should be either ZrH, (or) ZrH₂, (or) ZrH₃, (or) ZrH₄, but its formula is ZrH_{1.3 - 1.75}, i.e., it does not follow the valency rules.
 

${\mathrm{MgH}}_2 .....\mathrm{Covalent} \mathrm{hydrides}.....\mathrm{Stoichiometric} \mathrm{hydride}$