The relatively stable oxoMn(V) porphyrins generated by oxidation of Mn(III) porphyrins in alkaline solutions using peroxides are unambiguously identified as trans -dioxo species. Raman spectra revealed symmetric O=Mn-v=O stretching frequencies between 741 and 744 cm(-1) for five porphyrin complexes while the IR of dioxo-Mnv-tetrapentafluoroporphyrin [Mn-V(O2)(TPFPP)]displayed an antisymmetric stretch at 805 cm(-1). Both half-labeled and fully labeled 180-Mn-v porphyrin complexes were prepared, and the Raman and IR shifts correspond well with a linear, three-body O=Mn=O oscillator model. Terminal, monooxo formulations such as HO-Mn-v=O or five-coordinate Mn-v=O are excluded by the data. The H-1 NMR spectrum of [Mn-V(O2)(TMP)]-, which shows a single, sharp resonance for the two ortho methyl groups, is also consistent with octahedral coordination of D (4h) symmetry. The force constant F for the Mn=O double bond was calculated to be 454 N/m with a stretch-stretch force constant k = 67.2 N/m. Comparison with other Mn-O force constants revealed a very good agreement with Badger's rule spanning five Mn oxidation states. These dioxo-Mn-v porphyrins were shown to not exchange the oxo oxygens with bulk water solvent. Dioxo Mnv porphyrins, which are anionic species [Por-Mn-V(O)(2)h] are unreactive toward olefins in the presence of excess base but efficiently epoxidize cyclooctene upon protonation at -70 degrees C. These compounds are the only trans-dioxomanganese complexes of any type to be characterized as such, thus extending the known TL-bonding arrangements for first-row transition metals.