Interhalogen compounds ($X{X}_n^{\prime }$) follow specific stability rules:

1. The central atom ($X$) must be the larger, less electronegative halogen.
2. The central atom must be able to expand its octet using $d$-orbitals to accommodate multiple bonds.

• $FC{l}_3$ Fail: Here, Fluorine would be the central atom. But Fluorine is the most electronegative element (cannot be $X$) and lacks $d$-orbitals, so it cannot form more than one covalent bond. Thus, $FC{l}_3$ cannot exist, while $Cl{F}_3$ is perfectly stable.