Question

Why $Cl{F}_3$ exists but $FC{l}_3$ does not?

Answer 1

Chlorine has empty $d$-orbitals and it acquires excited state at the time of bonding when electron from $3p$-orbital are promoted to $3d$-orbital and thus can exhibit a covalency of three.

Fluorine cannot expand its octet due to the absence of empty $d$-orbitals in $2^{nd}$ energy shell. Thus it cannot exhibit covalency more than $1$.

Hence $Cl{F}_3$ exists but $F{Cl}_3$ does not.