We next obtained isomerization RCs where succinate is forced to remain bidentate along the RC to isolate the restrictive effect of bidentate coordination on isomerization and compare it to the RCs obtained with monodentate succinate (Figure ). Consistent with prior work , while bidentate succinate is more stable than its monodentate counterpart, we find that isomerization may not be feasible with fully bidentate succinate geometries for any of the three halides (Figure ). Comparison of isomerization RCs obtained with bidentate and monodentate isomers reveals that the structures on the bidentate RC for fluoride are energetically more favorable than those for chloride/bromide by ca. 2-3 kcal/mol (Figure ). isomers for fluoride which could explain why C-H fluorination is not observed in non-heme Fe(II) enzymes (Figure ). On the contrary, isomerization RCs of fluoride intermediates obtained as single-point calculations at chloride/bromide geometries exhibit comparable or even slightly reduced barriers when the Fe-F bond is lengthened (Supporting Information Figure ). These observations suggest that targeted modifications to residues in the enzyme active site to introduce residues that form halogen bonds with fluoride or create local electric fields that elongate Fe-F bond lengths could make selective C-H fluorination more favorable. This longer Fe-F bond would then be expected to make the isomerization and rearrangement of the fluoride proximal to the substrate radical more favorable.