We study two closely related isomerization RCs for monodentate succinate/Glu isomers of both Fe(IV)=O and V(IV)=O intermediates to compare the energetic differences for the two metals. The first RC connects the axial and equatorial oxo isomers as a function of NHis-M=O angle where both succinate and Glu remain monodentate during isomerization, and we note significant differences between Fe and V (Figure and Supporting Information Figure ). For Fe, isomerization from axial oxo to equatorial oxo is separated by a transition state (TS) with a high energy barrier of 10.4 kcal/mol for Fe(IV)=O (Figure ). The global minimum on the RC (NHis-Fe=O angle: 176°) corresponds to the axial oxo isomer and the local minimum (NHis-Fe=O angle: 90°) corresponds to the equatorial oxo isomer (Figure ). We characterize this TS by observing a single imaginary frequency along the angular RC mode (NHis-Fe=O angle: 124°).