Fifty years ago, in a master stroke, Enemark and Feltham chose to sidestep the problem of local oxidation states by assigning an effective d electron count n to metal nitrosyls. 9 Now known as the Enemark-Feltham electron count, n refers to the number of metal d electrons plus the number of NO p* electrons; thus 'Fe(II) + NO • ' corresponds to n = 6 + 1 = 7 and is denoted as {FeNO} . Despite the popularity of the notation, chemists have retained a strong interest in the oxidation state problem and have sought to assign oxidation states to both the metal and NO fragments in nitrosyl complexes. Unfortunately, density functional theory, the major theoretical tool for such studies, suffers from several pitfalls. To start with, the DFT description generally does not correspond to a pure spin state, but incorporates contamination from multiple states. In addition, different exchange-correlation functionals provide disturbingly divergent descriptions of metal-ligand covalence and of spin-state energetics. In the face of these challenges, chemists have increasingly resorted to a socalled spectroscopically calibrated approach, i.e., a combination of several spectroscopic methods and DFT calculations, to come up with local oxidation states in nitrosyl complexes. Modern multiconfigurational methods and orbital localization schemes provide a neat alternative to these somewhat ad hoc approaches, as we illustrated recently in a study of transition metal corroles. Here we present a state-of-the-art DMRG/CASSCF-CASPT2 study of seven paradigmatic FeNO porphyrin derivatives spanning {FeNO} electron counts (Scheme 1). Two {FeNO} 7 systems were examined (see relevant experimental papers ): Fe[P](NO), i.e., a five-coordinate nitrosylheme, and its six-coordinate analogue Fe[P](NO)(ImH), where P is unsubstituted porphyrin and ImH is imidazole, a model for the amino acid histidine. Four oxidized {FeNO} 6 systems, so-called met-heme nitrosyl derivatives, were examined (see relevant experimental papers ): (iii) {Fe[P](NO)} + , (iv) {Fe[P](NO)(ImH)} + , Fe[P](NO)(NO2), and Fe[P](NO)(SMe). Finally, one reduced {FeNO} 8 system, a heme-nitroxide derivative, was examined (see relevant experimental papers ): {Fe[P](NO)} -. The present calculations provide a definitive resolution of several long-standing questions, including (i) the spin state energetics of the major classes of FeNO porphyrins, (ii) their spin density profiles (where applicable),