`ClF_(3)` exists, but `FCl_(3)` does not.

Explanation

Cl atom has empty d-orbitals. During obnd formation the electrons from 3p-orbitals are promoted to 3d-orbitals.

In the first excited state, Cl-atom can exhibit a covalency of three. Hence `ClF_(3)` is possible. F-atom cannot expand its octet due to absence of empty d-orbitals in 2nd enegy level. hence it cannot exhibit covalency more than 1. therefore `FCl_(3)` is not possible.