Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear FeIII and MnIII complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N′ -tertbutylureaylato)-N-ethyl]aminato([H31]3-), which creates a protective hydrogen bond cavity around the MIII-O(H) units (MIII = Fe and Mn). The MIII-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the MIII-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [FeIIIH 31(O)]2- complex also can be made using sulfoxides. These findings support the proposal of a high valent MIV-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [MIIIH31(O)]2- complexes come directly from the cleavage of dioxygen: for [FeIIIH 31(O)]2- the v(Fe-16O) = 671 cm-1, which shifts 26 cm-1 in [FeIIIH31(18O)] 2- (v(Fe-18O) = 645 cm-1); a v(Mn- 16O) = 700 cm-1 was observed for [MnIIIH 31(16O)]2-, which shifts to 672 cm -1 in the Mn-18O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) Å in [FeIIIH 31(O)]2-, while a longer bond is found in [Fe IIIH31(OH)]- (Fe-O at 1.926(2) Å); a similar trend was found for the MnIII-O(H) complexes, where a Mn-O distance of 1.771(5) Å is observed for [MnIIIH 31(O)]2- and 1.873(2) Å for [MnIIIH 31(OH)]-. Strong intramolecular hydrogen bonds between the urea NH groups of [H31]3- and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single σ-bond exists between the MIII centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace π-bonds in stabilizing oxometal complexes.