Question

The limiting composition of f - block hydraides????

Answer 1

Unlike d block elements (transition elements) the f block elements form hydrides with fixed composition of MH2 or MH3. But d block elements will form non stoichiometric hydrides without any specific composition. (The composition of hydrides will change with respect to temperature and pressure etc).
But both d block element hydrides and f block elements hydrides are interstitial hydrides (The hydroges will be drapped in between the spaces of metal atom)