Transition elements form interstitial compounds. Give one reason?

Hint: As we know, the transition metals are found in between the periodic table. All the transition metals show variable oxidation state due to the presence of vacant d- orbitals. As we know that, these are present between s- block and p- block so these have comparable size.

Complete step by step solution
The transition metals are found in between the main block. The transition metals have large size because these are having d- orbitals which contain unpaired electrons except zinc, cadmium and mercury. So, due to these unpaired electrons the transition metal forms variable oxidation states. Due to the variable oxidation states, the small atoms of carbon nitrogen boron oxygen hydrogen can be trapped into the vacant spaces in the crystal lattice. This vacant space is known as voids or interstitial position. Void is having enough space to contain the small atoms and form interstitial compounds.

The transitions metal crystals also have defects. The defects arise due to the vacant lattice site or occupied interstitial sites. Due to the variable oxidation states, size of metal also affects. The more the electrons in the outermost shell of metals, the more it can show variable oxidation states. As the oxidation state increases the size of the metal decreases and hence that size matches the interstitial size and therefore metal jumps to interstitial position leaving its position vacant. That arises defects.
Therefore, most of the transition metal forms interstitial compounds.

Note:
All types of defects are shown by mostly transition metals such as Schottky defect, Frenkel defect, metal excess, metal deficiency defects.