The F-F bond dissociation enthalpy ($159\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) is lower than that of Cl-Cl ($243\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). The main reason is

1. Athe F-F bond is ionic in nature
2. BF has vacant d orbitals that weaken the bond
3. Cthe F atom is larger than the Cl atom
4. Dstrong repulsion between the lone pairs on the two small, closely bound F atoms