The EPR spectra of {FeNO} 7 iron nitrosyls are of interest as models for nitrosylated nonheme proteins which exhibit the unusual S = 3 2 spin state. Few models of such S = 3 2 species are known, although [Fe(edta)NO] 2− adopts this spin behavior. The present study examines [Fe IIL] and [Fe IIL(NO)] complexes of polyaminopolycarboxylates and pyridylmethylamines which are derived from nta 3− and edta 4− as models of such systems. The series of L = nta 3− (nitrilotriacetate), uedda 2− (N,N′-ethylenediaminediacetate), pida 2− 2-pyridylmethyliminodiacetate), tpa (tris(2-pyridylmethyl)amine), tpen (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine), and edta 4− (ethylenediaminetetraacetate) was chosen to provide ligands of increasing ligand field strength for both six-coordinate and seven-coordinate [FeL(NO)] complexes in order to investigate the effect on the net spin states of the nitrosyls. Electrochemical methods (CV, DPP) show that the dominant complexes near 1:1 ligand/Fe II ratios are [Fe(pida)(H 2O) 2] and [Fe(uedda)(H 2O) 2] having E 1 2 values of 0.39 V and 0.26 V vs NHE ( μ = 0.1, T = 22°C). [Fe(tpa)(H 2O)Cl] + and [Fe(tpen)] 2+ have waves at 0.547 V and 0.84 V. Bis complexes occur at 20:1 ratios for pida 2− and uedda 2−. [Fe IIL(NO)] complexes are spontaneously formed by admitting NO to Ar purged Fe IIL solutions. EPR spectra of frozen samples (77–105K) show that [Fe(nta)NO] − (g = 4.36, 402, 2.00), [Fe(pida)(H 2O)(NO)] (g = 4.02 (axial), 2.00), [Fe(tpa)Cl(NO)] + (g = 4.00, 2.00) are S = 3 2 complexes. [Fe(tpen)NO] 2+ (g = 2.03, 1.97, 1.96) is a low-spin S = 1 2 complex with no N-shf coupling, suggestive of a seven-coordinate structural analogue of the [Fe(edta)NO] 2−, S = 3 2 complex. MO diagrams for six- and seven-coordinate [FeL(NO)] complexes as a function of increasing ligand field strength, which incorporate the spin-polarization effect for weak fields, are presented to explain the change from S = 3 2 to S = 1 2 of the nonheme protein model complexes. The strong-field limit yields the Enemark-Feltham order. Cyclic voltammetry and differential pulse polarography studies support the prior Rhodes-Barley-Meyer conclusion for [Fe(edta)NO] 2− that, for the [FeL(NO)] complexes, the oxidation [Fe IIL(NO) ] −1 e→ [ Fe IIL(NO +)] occurs coincidentally at the same, or nearly the same, potential as their [ Fe IIL(H 2 O)] −1 e[ −→ [ Fe IIIL(H 2 O)] complex. The [Fe IIL(NO)] complexes of nta 3−, pida 2−, and edta 4− are electrochemically silent at glassy carbon, whereas those of tpa and tpen are electrochemically reversible. Consistent with the MO predictions, six-coordinate S = 1 2 {FeNO} 7 complexes have both metal d xz and ligand NO N character in the HOMO, and exhibit N-shf in the EPR spectrum. Seven-coordinate S = 1 2 {FeNO} 7 complexes have purely metal-based d xy HOMO's and exhibit no N-shf coupling. The absence of stable CO adducts for many nonheme proteins and their model complexes is explained by the MO orders and the reduction of spin polarization as a stabilizing factor.