We first study the isomerization RC that connects equatorial and axial halide isomers with equatorial oxo and monodentate succinate via an RC described by the NHis-Fe-X angle formed with the His ligand that is trans to the axial moiety (Figure ). Comparison of this isomerization RC across three halides reveals differences in the stabilities of the equatorial halide configuration, which is not a minimum on the RC, but which we refer to as the equatorial isomer (Figure ). This equatorial fluoride isomer is more favorable than the equatorial chloride and bromide isomers by ca. 2 kcal/mol and 3 kcal/mol, respectively (Figure ). While the equatorial halide isomer is energetically more stable than the axial halide isomer for all three halides, their energies are more comparable for larger halides, i.e., the equatorial isomer is strongly preferred over the axial isomer by 4.5 kcal/mol for fluoride but only by 2.5 kcal/mol for bromide (Figure ). We also find that the lowest-energy structure positions the halide tilted out of the equatorial plane and is observed at increasingly obtuse RC angles with increasing halide size, i.e., the lowest-energy NHis-Fe-X angle is 137°, 140°, and 147° in fluoride, chloride, and bromide intermediates, respectively (Figure ).