The reactions of the nitrogen Lewis bases (13) 1-methylimiclazole (1-Melm), pyridine (Py), and NH3 as gases with sublimed layers containing the 5-coordinate nitrito iron(III)-porphyrinato complexes Fe(Por)(eta(1)-ONO) (1) are described (Por = meso-tetraphenyl-porphyrinato or meso-tetra-p-tolyl-porphyrinato dianions). In situ FTIR and optical spectra are used to characterize the formation of the 6-coordinate nitro complexes formed by the reaction of I with B = I-Melm, Py, or NH3. These represent the first examples of 6-coordinate amino-nitro complexes with sterically unprotected iron-porphyrins. The interaction of ammonia with Fe(Por)(ONO) at 140 K initially led to the nitrito species Fe(Por)(NH3)(eta(1)-ONO), and this species isomerized to the nitro complexes Fe(Por)(NH3)(eta(1)-NO2) upon warming to 180 K. When the latter were warmed to room temperature under intense pumping, the initial nitrito complexes Fe(Por)(eta(1)-ONO) were restored. Assignments of vibrational frequencies for the coordinated nitro group in 6-coordinate iron-porphyrin complexes are given and confirmed using N-15-labeled nitrogen dioxide to identify characteristic infrared bands. For M(Por)(B)(NO2) complexes (M = Fe or Co), an inverse correlation between the net charge transfer from the axial ligand B to the nitro group and the value of Delta v = v(a)(NO2) - v(s)(NO2) is proposed: These observations are discussed in the context of growing interest in potential physiological roles of nitrite ion reactions with ferro- and ferri-heme proteins.