Interstitial compounds are formed when small atoms like H, C, or N are trapped inside the crystal lattices of transition metals.
The principal physical and chemical characteristics of interstitial compounds are:
(i) They have high melting points, higher than those of pure metals.
(ii) They are very hard, and some borides approach diamond in hardness.
(iii) They retain metallic conductivity.
(iv) They are chemically inert.
(v) They are usually non-stoichiometric.
Therefore, the statement that they are very soft and ionic in nature is incorrect. They are very hard and are neither typically ionic nor covalent.
Answer: They are very soft and ionic in nature.