Fel3 does not exist because of I−. Being a very good reducing agent, I−reduces Fe3+ to Fe2+. 2Fe3++2I−⟶2Fe2++I2 Reducing nature order I>Br>Cl>F Fe3++e−⟶Fe2+E∘=0.77V I2+2e−⟶2I−E∘=−0.54V As standard reduction potential value of Fe3+ is more than I2, so it is reduced to Fe2+. Hence, except Fel3, all other halogens can form iron halide compounds.