Using the calibration curve, one can also estimate the π* occupancies of the complexes, although the results should be viewed qualitatively, as we found a significant downshift of the data points from the calibration curve. This behavior is also found in other nonheme complexes but to a smaller extent (unpublished). Based on the NO vibrational frequency, the occupancies should be 0.95, 1.35, and 1.7 for {Fe[P](NO)} + , Fe[P](NO), and {Fe[P](NO)} -, respectively. However, based on the NO bond distance, the occupancies are 0.90, 1.15, and 1.4, respectively. These results are in moderate agreement with those obtained via the DMRG-CASSCF-based resonance form analysis outlined above. The analysis suggests that the NO's in both {Fe[P](NO)} + and Fe[P](NO) are best approximated as NO 0 , while the one in {Fe[P](NO)} -is somewhere around NO -0.5 . On the other hand, this analysis is inconsistent with the result that both {Fe [ occupancies, i.e., especially the metal-NO + , metal-NO 0 , metal-NO -, and metal-NO 2-resonance forms. For the {FeNO} 7 and {FeNO} 6 complexes studied, the wave function in each case indicated a dominant NO 0 resonance form. For the {FeNO} 8 complex {Fe[P](NO)} -, a similar exercise indicated a resonance hybrid, Fe(I)-NO 0 « Fe(II)-NO -, with both resonance forms making comparable contributions of 44 ± 6%. These findings contradict a number of common formulations for nitrosyl complexes, most notably Fe(II)-NO + for {FeNO} 6 heme-nitrosyl systems, but are consonant with an L-edge XAS study of an octahedral low-spin nonheme {FeNO} 6 complex, which the authors formulated as Fe(III)-NO . To what extent the present conclusions are transferable to high-spin nonheme iron nitrosyls remains a fascinating question at this point.