Use of the box method to determine lowest term symbol for lanthanides but not iron

I am trying to compare the predicted magnetic moment for Fe(CN)6 using the spin-only formula magnetic moment = $\sqrt{4S(S+1) + L(L+1)}$.


However, if I use the box method for Fe(CN)6 to predict the lowest term symbol of Fe3+, I get 6S. If L = 1, however, then the predicted value of the magnetic moment is similar to the experimentally derived value. 


Can we not use this method because in the complex the octahedral field changes the energy levels of the d orbitals?


If this is the case, then why can we use the box formula to determine the magnetic moments of lanthanides using the formula magnetic moment = g$\sqrt{J(J+1)}$.


