In {Fe[P](NO)} -, the contribution of [π*(NO)} 1 configurations is dramatically lower, with that of [π*(NO)} 2 configurations correspondingly higher. Accordingly, to a first approximation, {Fe[P](NO)} -appears best described as a resonance hybrid: Fe(I)-NO 0 « Fe(II)-NO -. Going from {FeNO} 7 to {FeNO} 8 , the reduction thus is not entirely metal-centered, as speculated above, but also significantly on the NO. Such a description is largely in accord with earlier theoretical studies on low-spin {FeNO} 8 species, including one by one of us. 43 (d) Insights from NO bond distances and vibrational frequencies. Given that the NO bond distance and vibrational frequency are known to qualitatively vary as a function of NO π* occupancy, we looked into the possibility of a semiquantitative correlation. Toward that end, we optimized and determined the vibrational frequency of NO as an isolated diatomic, with the π* occupation varying from 0 to 2 (i.e., from NO + to NO -). Fractional orbital occupations were also employed in this exercise. An essentially linear relationship was found to exist among N-O distance, vibrational frequency, and π* occupancy. As hoped for, the N-O distances and vibrational frequencies of the FeNO porphyrins studied also appeared to follow the same relationship, allowing an empirical read-out of NO π* occupancies in the different molecules (Figure ). Note that the couplings between the NO vibration and other vibrational modes are small. In all complexes, the NO bond distance ranges from 1.153 Å to 1.203 Å, but never exceeds the value of 1.213 Å corresponding to NO -0.5 (or the occupancy of 1.5 of the NO(π*) orbitals).