As models for a reaction intermediate in the catalytic cycle of dissimilatory nitrite reductases, we have examined one-electron oxidation of nitrosyl-iron(II) complexes of octaethylporphyrin (OEP, 1a), oxooctaethylchlorin (oxo-OEC, 1b), and dioxooctaethylisobacteriochlorin (dioxo-OEiBC, 1c). While (OEP)Fe-III(NO) 2a is obtained in the oxidation of 1a, the oxidation products of 1b and 1c afford absorption spectra characteristic of chlorin and isobacteriochlorin pi-cation radicals. The formation of the pi-cation radical complexes (2b and 2c) is confirmed by a variety of methods including electronic absorption, ESR, NMR and IR spectroscopies. The presence of NO in 2b and 2c as the fifth ligand is evidenced by the observation of nu(15NO) bands at 1686 and 1699 cm(-1), respectively. These results are rationalized by the stabilization of the iron d orbital levels in 2b and 2c. Sixth-ligand effects on the electronic structures of the oxidation products (2a-c) have also been investigated. Ligation of N-methylimidazole (N-MeIm) to 2b and 2c causes valence isomerization to give Fe-II(NO+)-(N-MeIm) complexes (3b and 3c) as well as (OEP)Fe-II(NO+)(N-MeIm) (3a). Although the six-coordinated imidazole adducts (3a-c) are formulated as Fe-II(NO+)(N-MeIm), the oxo derivatives (3b and 3c) readily release the NO ligand in the presence of an additional 1 mol equiv of N-MeIm : 3a is relatively stable under the same condition.