RCs also reveals considerable differences between hydroxylases and halogenases. In contrast to halogenases, OH isomerization in hydroxylases can take place through two pathways: one with HBs between OH and Glu, and the other without hydrogen bonding interactions. While the latter RC is qualitatively similar to the OH isomerization energy landscape observed in halogenases, it is likely that the former RC (i.e., with HBs between OH and Glu) would be preferred due to its overall favorable energetics. For both hydroxylases and halogenases overall, angular RCs and energy landscapes differ significantly between Fe and their V mimics, demonstrating that conclusions on the ease of active site isomerization will be metal-dependent and reinforcing the limits of using vanadyl species to understand catalysis in these enzyme classes. The formation of monodentate succinate and/or monodentate Glu is likely necessary for isomerization of M(IV)=O intermediates. However, the most energetically favorable ferryl isomers prefer a bidentate Glu or bidentate succinate (Figure ). In order to quantify the differences in the binding strength of monodentate and bidentate succinate/Glu to the metal centers, i.e., Fe and V, we perform Mayer bond order analysis, a quantum mechanical assessment of bond order.