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Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [Fe-V(N)(TPP)] (1, TPP2- = tetraphenylporphyrinato), and [Fe-V(N)(cyclam-ac)](+) (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and Fe-57 Mossbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [Fe-V(N)(MePy(2)tacn)](PF6)(2) (3, MePy(2)tacn = methyl-N',N ''-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe-V(N){PhB(t-BuIm)(3)}](+) (4, PhB((t)BuIm)(3)(-) = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [Fe-V(O)(TAML)](-) (5, TAML(4-) = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferry] unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and-oxo complexes 1-3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g(parallel to) < g(perpendicular to) <= 2 were measured for them, and their g(parallel to) and g(perpendicular to) values were found to obey a simple relation of g(perpendicular to)(2) + (2 - g(parallel to))(2) = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g(parallel to) < 2 < g(perpendicular to). Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and-oxo complexes feature electronic structures akin to those found for complexes 1-3 and 5. Therefore, the characteristic EPR signals determined for 1-3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.
