The spectroscopic properties and electronic structure of an Fe-2(III,IV) bis-mu-oxo complex, [Fe2O2(5-Et-3-TPA)(2)](ClO4)(3) where 5-Et-3-TPA = tris(5-ethyl-2-pyridylmethyl)amine, are explored to determine the molecular origins of the unique electronic and geometric features of the Fe2O2 diamond core. Low-temperature magnetic circular dichroism (MCD) allows the two features in the broad absorption envelope (4000-30000 cm(-1)) to be resolved into 13 transitions. Their C/D ratios and transition polarizations from variable temperature-variable field MCD saturation behavior indicate that these divide into three types of electronic transitions; t(2) --> t(2)* involving excitations between metal-based orbitals with pi Fe-O overlap (4000-10000 cm(-1)), t(2)/t(2)* --> e involving excitations to metal-based orbitals with sigma Fe-O overlap (12500-17000 cm(-1)) and LMCT (17000-30000 cm(-1)) and allows transition assignments and calibration of density functional calculations. Resonance Raman profiles show the C-2h geometric distortion of the Fe2O2 core results in different stretching force constants for adjacent Fe-O bonds (k(str)(Fe-O-long) = 1.66 and k(str)(Fe-O-short) = 2.72 mdyn/Angstrom) and a small (similar to20%) difference in bond strength between adjacent Fe-O bonds. The three singly occupied pi*-metal-based orbitals form strong superexchange pathways which lead to the valence delocalization and the S = 3/2 ground state. These orbitals are key to the observed reactivity of this complex as they overlap with the substrate C-H bonding orbital in the best trajectory for hydrogen atom abstraction. The electronic structure implications of these results for the high-valent enzyme intermediates X and Q are discussed.