The second RC corresponds to isomerization between the axial and equatorial positions for the oxo while the halide remains equatorial and succinate remains monodentate (Figure ). We describe this RC as a function of NHis-Fe=O angle formed with the His ligand that is trans to the axial moiety (Figure ). We find that the energies of both minima corresponding to axial oxo and equatorial oxo isomers differ by less than 1 kcal/mol between the smaller fluoride and larger chloride/bromide intermediates (Figure ). Furthermore, the angular RCs are broadly consistent across all halides with a barrier of ca. 8 kcal/mol for oxo isomerization (Figure ). The high-energy structures along these RCs are confirmed to be transition states with a single imaginary frequency along the angular RC mode (Supporting Information Table ).