In a previous study, we analyzed the electronic structure of S ) 3/2 {FeNO}7 model complexes [Brown et al. J. Am. Chem. Soc. 1995, 117, 715-732]. The combined spectroscopic data and SCF-XRSW electronic structure calculations are best described in terms of FeIII (S ) 5/2) antiferromagnetically coupled to NO- (S ) 1). Many nitrosyl derivatives of non-heme iron enzymes have spectroscopic properties similar to those of these model complexes. These NO derivatives can serve as stable analogues of highly labile oxygen intermediates. It is thus essential to establish a reliable density functional theory (DFT) methodology for the geometry and energetics of {FeNO}7 complexes, based on detailed experimental data. This methodology can then be extended to the study of {FeO2}8 complexes, followed by investigations into the reaction mechanisms of non-heme iron enzymes. Here, we have used the model complex Fe- (Me3TACN)(NO)(N3)2 as an experimental marker and determined that a pure density functional BP86 with 10% hybrid character and a mixed triple-ú/double-ú basis set lead to agreement between experimental and computational data. This methodology is then applied to optimize the hypothetical Fe(Me3TACN)(O2)- (N3)2 complex, where the NO moiety is replaced by O2. The main geometric differences are an elongated Fe-O2 bond and a steeper Fe-O-O angle in the {FeO2}8 complex. The electronic structure of {FeO2}8 corresponds to FeIII (S ) 5/2) antiferromagnetically coupled to O2 - (S ) 1/2), and, consistent with the extended bond length, the {FeO2}8 unit has only one FeIII-O2 - bonding interaction, while the {FeNO}7 unit has both ó and ð type FeIII-NO- bonds. This is in agreement with experiment as NO forms a more stable FeIIINO- adduct relative to O2 -. Although NO is, in fact, harder to reduce, the resultant NO- species forms a more stable bond to FeIII relative to O2 - due to the different bonding interactions.