Question

why do transition elements form a number of interstitial compounds

Answer 1

here..it is your answer 

Answer 2

Due to the large sizes of transition elements, they have lots of interstitial sites and can trap smaller elements such as H,C,N etc... Such compounds are called interstitial compounds.

Transition metals often have close packed (ccp,hcp) lattices that have N oct interstices (holes!) for N close packed metal atoms. Metals to the left of the PT (1st row TM Ti, V not Cr or Fe) are the largest and the holes can accommodate a C or a N atom to give interstitial cmpds that are readily formed when the two elements are heated together at high T. The best known is tungsten carbide WC (Wikipedia must have an entry) that is extremely hard and used in steel cutting tools, armor and jewelry. Tungsten nitride WN is also known so interstitial cmpds do not obey conventional bonding rules! Very simply the C/N enters the hole and gives up some of its valence e⁻s to the conduction band thus increasing the lattice energy (and often the conductivity). This is a gross over simplication for example WC forms an hcp like lattice with alternate W and C layers. And I think W is ccp to start with (check)The cmpds are often non stoichiometric (Wikipedia?) M is stable, MC is stable so it is not surprising that say MC0.75 is stable. Oxides MO are partly ionic, and partly metallic (interstitial).The most remarkable cmpd in this category does not have a TM. It is MgB2 (Wikipedia?) magnesium boride. It has unique (?) superconducting properties that may very soon create a bit of a revolution in areas such as MRI, electrical transmission, and even bullet trains.


hope this helps you.
REGARDS