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 Fe-III and Mn-III complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato [H(3)1](3-)), which creates a protective hydrogen bond cavity around the M-III-O(H) units (M-III = Fe and Mn). The M-III-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M-III-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M-IV-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-O-16) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1 (180)](2-) (nu(Fe-O-18) = 645 cm(-1)); a nu(Mn-O-16) = 700 cm(-1) was observed for [Mn(III)H(3)1(O-16)](2-), which shifts to 672 cm(-1) in the Mn-O-18 isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) Angstrom in [Fe-III H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) Angstrom); a similar trend was found for the Mn-III-O(H) complexes, where a Mn-O distance of 1.771(5) Angstrom is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) Angstrom for [Mn-III-H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](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 a-bond exists between the M-III centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.