Interstitial compounds are formed when small atoms like $\text{H}$, $\text{C}$ or $\text{N}$ are trapped inside the crystal lattices of metals. The principal physical and chemical characteristics of these compounds are: (i) They have high melting points, higher than those of pure metals. (ii) They are very hard, some borides approach diamond in hardness. (iii) They retain metallic conductivity. (iv) They are chemically inert. Therefore, the statement 'They are chemically reactive' is incorrect.