Examination of optimized geometries of halide intermediates in their ground spin states shows that iron-halide bond lengths increase on average with halide size from 1.81 Å (F) to 2.25 Å (Cl) to 2.41 Å (Br) in Fe(IV)=O and Fe(III)-OH intermediates (Supporting Information Table ). To assess whether this increase in bond lengths is solely due to differences in the size of the halides, we considered bond lengths scaled by the covalent radii of participating atoms (Supporting Information Table ). The scaled Fe-F bond is shorter than Fe-Cl (0.84 vs 0.90), while scaled Fe-Cl and Fe-Br bond lengths are comparable. The shorter scaled and unscaled Fe-F bonds suggest that it would be harder for the substrate to approach the halide in the oxo or hydroxo intermediates, which could explain the challenges with C-H fluorination. All other metal-ligand bond lengths are mostly comparable across fluoride, chloride, and bromide intermediates (Supporting Information Table ). The Fe-Cl and Fe-Br distances observed in optimized geometries of Fe(II)-H2O intermediates, i.e., 2.38 Å and 2.54 Å, respectively, are roughly comparable to those observed in the crystal structures of BesD and SyrB2, suggesting that the shortened Fe-F bond lengths are treated reliably by DFT (Supporting Information Table ).