We have synthesized and characterized, using X-ray crystallographic, spectroscopic, and computational techniques, a six-coordinate diazide Fe3+ complex, LFe(N-3)(2) (where L is the tetradentate ligand 7-diisopropyl-1,4,7-triazacyclononane-1-acetic acid), that serves as a model of the azide adducts of Fe3+ superoxide dismutase (Fe3+SOD). While previous spectroscopic studies revealed that two distinct azide-bound Fe3+SOD species can be obtained at cryogenic temperatures depending on protein and azide concentrations, the number of azide ligands coordinated to the Fe3+ ion in each species has been the subject of some controversy. In the case of LFe(N-3)(2), the electronic absorption and magnetic circular dichroism spectra are dominated by two broad features centered at 21 500 cm(-1) (epsilon approximate to 4000 M-1 cm(-1)) and similar to 30 300 cm(-1) (epsilon approximate to 7400 M-1 cm(-1)) attributed to N-3 -> Fe3+ charge transfer (CT) transitions. A normal coordinate analysis of resonance Raman (RR) data obtained for LFe(N-3)(2) indicates that the vibrational features at 363 and 403 cm(-1) correspond to the Fe-N-3 stretching modes (VFe-N3) associated with the two different azide ligands and yields Fe-N-3 force constants of 1.170 and 1.275 mdyne/angstrom, respectively. RR excitation profile data obtained with laser excitation between 16 000 and 22 000 cm(-1) reveal that the VFe-N3 modes at 363 and 403 cm(-1) are preferentially enhanced upon excitation in resonance with the N-3(-) -> Fe3+ CT transitions at lower and higher energies, respectively. Consistent with this result, density functional theory electronic structure calculations predict a larger stabilization of the molecular orbitals of the more strongly bound azide due to increased a-symmetry orbital overlap with the Fe 3d orbitals, thus yielding higher N-3(-) -> Fe3+ CT transition energies. Comparison of our data obtained for LFe(N-3)(2) with those reported previously for the two azide adducts of Fe3+SOD provides compelling evidence that a single azide is coordinated to the Fe3+ center in each protein species.