The electronic origins of the magnetic signatures of [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3), where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon (17)O substitution in the Fe(2)O(2) core, and an unusually large, positive zero-field splitting parameter, D = 38 +/- 3 cm(-1). Density functional calculations identify the anisotropic (17)O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe(2)O(2) core in the three singly occupied molecular orbitals of the S = (3)/(2) ground state. The near-axial g-matrix arises from DeltaS = 0 spin-orbit mixing between the singly and doubly occupied d(pi) orbitals of the iron d-manifold. The large D is due to DeltaS = +/-1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe(2)O(2) core, and define the low-lying valence orbitals responsible for reactivity.