These studies highlighted the dramatic reactivity enhancement which could be achieved by varying the primary and the secondary coordination spheres around the metal-oxo center. Indeed, some Mn-oxo systems have demonstrated exceptional chemical (and in some cases even catalytic) reactivities and selectivities, rivaling natural metalloenzymes in both OAT and hydrogen atom abstraction (HAA) reactions. Due to their highly reactive nature, only a handful of XRD structures of monomeric Mn-oxo complexes have been obtained. Most of them feature Mn(V)-oxo species which are sufficiently stabilized by means of strongly electron donating ligands, such as macrocyclic porphyrins and corrolazines, or chelating dialkoxy-diamido and tetra-amido ligands. In addition, several crystal structures of Mn(III)-oxo species supported by tripodal ligands were reported by Borovik and others. In addition to the electronic stabilization, gained due to shifting from a tetragonal to a trigonal ligand field (an effect well-known for d and d 3 metal-oxos) , these tris-urea containing ligands also stabilized the Mn(III)-oxo moiety through three intramolecular hydrogen bonds. However, the corresponding Mn(IV)-oxo species (complex IV, Fig. ), could only be obtained in solution at -80°C, and was too reactive for isolation in the solid state. In fact, the only known example of a crystallographically characterized Mn(IV)-oxo complex has been reported by Nocera in 2018 (complex V, Fig. ). In this case, the necessary stabilization was achieved by the combination of a pseudo-tetrahedral C3v symmetry with the bulky and highly electron-donating ditox ligands.