Nonheme iron enzymes perform diverse and important functions in biochemistry. The active form of these enzymes comprises the ferryl, oxidoiron(1V), [FeO](2+) unit. In enzymes, this unit is in the high -spin, quintet, S = 2, ground Exp State, while many synthetic model compounds exist in the spin triplet, S = 1, ground state. Recently, however, Que and co-workers reported an oxidoiron(IV) complex with a quintet ground state, [FeO(TMG(3)tren)](OTf)(2), where TMG(3)tren = 1,1, 1-ttis {2-[N2-(1,1,3,3 -tetramethylguanidino)]ethyl}amine and OTf = CF3SO3-. The trigonal geometry imposed by this ligand, as opposed to the tetragonal geometry of earlier model complexes, favors the high -spin ground state. Although [FeO(TMG(3)tren)](2+) has been earlier probed by magnetic circular dichroism (MCD) and Mossbauer spectroscopies, the technique of high -frequency and -field electron paramagnetic resonance (HEEPR) is superior for describing the electronic structure of the iron(IV) center because of its ability to establish directly the spin -Hamiltonian parameters of high -spin metal centers -with high precision. Herein we describe HFEPR studies on [FeO(TMG(3)tren)](OTf)(2) generated in situ and confirm the S = 2 ground state with the following parameters: D = +4.940(5) cm(-1), E = 0.000(5), B-4(0) = -14(1) x 10(-4) cm(-1), g(perpendicular to) = 2.006(2), and g(parallel to) = 2.03(2): Extraction of a fourth -order spin -Hamiltonian parameter is unusual for HFEPR and impossible by other techniques. These experimental results are combined with state-of-the-art computational studies along with previous structural and spectroscopic results to provide a complete, picture of the electronic structure of this biomimetic complex. Specifically, the calculations reproduce well the spin -Hamiltonian parameters of the complex, provide a satisfying geometrical picture of the S = 2 oxidoiron(IV) moiety, and demonstrate that the TMG(3)tren is as "innocent" ligand.