To further understand the differences between Fe intermediates and their V-based mimics, we studied active site isomerization of Fe(IV)=O with monodentate succinate and monodentate Glu between axial and equatorial oxo isomers. We observed that the local and global minima for Fe(IV)=O isomers are separated by a transition state with a large barrier of ca. 10 kcal/mol that increases by 3 kcal/mol when Glu is allowed to become bidentate. This suggests that favorable interactions in the protein environment, e.g., hydrogen bonding interactions with Glu, could help reduce the isomerization barrier by forcing Glu to remain monodentate. Comparison of these isomerization barriers to those observed for halogenases showed that isomerization is more difficult in hydroxylases likely due to the presence of a larger Glu/Asp residue that could be forced into unfavorable orientations along the isomerization RC. Study of the corresponding isomerization RC for V(IV)=O reveals a significantly different barrierless energy landscape with only one minimum energy structure where oxo is located midway between the axial position and the equatorial plane. The Fe(IV)=O isomerization RC with bidentate Glu has more energetically favorable structures relative to RCs with bidentate succinate or those with monodentate structures, an effect not captured by the vanadyl mimics. Although more modest, differences were also observed in Fe(III)-OH versus V(III)-OH isomerization. Finally, we analyzed the relative bond strengths of monodentate and bidentate Glu/succinate for Fe and V in relation to the binding strength of the bidentate aKG and found that Glu and succinate bidentate binding is much weaker than aKG for both Fe and V. While differences between Fe and V were smaller, we generally found that the LS V intermediates especially for O2-bound intermediates more strongly than the Fe counterparts.