137.

What are interstitial compounds? Why are such compounds well known for transition metals?

Interstitial compounds are those which are formed when small atoms like H, C, N, B etc. are trapped inside the crystal lattices of metals. They are generally non-stoichiometric and neither typically ionic nor covalent.
Most of transition metals form interstitial compounds with small non-metal atoms such as hydrogen, boron, carbon and nitrogen. These small atoms enter into the void sites between the packed atoms of crystalline transition metals and form chemical bonds with transition metals. For Example, steel and cast iron become hard by forming interstitial compound with carbon.
The existence of vacant (n – 1) d orbitals in transition elements and their ability to make bonds with trapped small atoms in the main cause of interstitial compound formation. Other examples are : VH_0.56, TiH_1.7 Some main characteristics of these compounds are:
(i) They have high melting and boiling points, higher than those of pure metals.
(ii) They are very hard. Some borides of transition elements approach diamond in hardness.
(iii) They are chemically inert but retain metallic conductivity.