Why `ClF_3` exists, but `FCl_3` does not exist ?

Explanation

Cl can extend its covalency upto three. This is possible because of the vacant d-orbitals in the valence shell of chlorine atom. Cl (Z = 17) = `1s^(2) 2s^(-) 2p^(6) 3s^(2) 3ps^(5)`
Ground state electronic configuration of chlorine atom Excited state electronic configuration of chlorine atom F-atom cannot extend hs covalency beyond one. This is because of the absence of d- orbitals in the valence shell of F-atom.