Metallic or Non-stoichiometric (or Interstitial) Hydrides

Metallic Hydrides

Many d-block and f-block elements form this third class of hydride, though notably Groups 7, 8 and 9 do not, and from Group 6 only chromium forms a hydride ($CrH$).

Key properties:

• They conduct heat and electricity, though less efficiently than the parent metal.
• They are almost always non-stoichiometric and hydrogen-deficient — for example $LaH_{2.87}$, $YbH_{2.55}$, $TiH_{1.5-1.8}$, $ZrH_{1.3-1.75}$, $VH_{0.56}$, $NiH_{0.6-0.7}$, $PdH_{0.6-0.8}$. The law of constant composition simply does not apply to them.

Structure: it was once thought that hydrogen merely squeezes into the interstitial gaps of the metal lattice without changing its type — hence the older name "interstitial hydride." More recent studies show that, apart from the hydrides of Ni, Pd, Ce and Ac, the lattice actually does change from that of the parent metal. …