Nitrosamines are well-known for their toxic and carcinogenic properties. The metabolic activation of nitrosamines occurs via interaction with the heme-containing cytochrome P450 enzymes. We report the preparation and structural characterization of a number of nitrosamine adducts of synthetic iron porphyrins. The reactions of the cations [(por)Fe(THF)(2)]ClO4 (por = TPP, UP, OEP) with dialkylnitrosamines (R2NNO; R-2 = Me-2, Et-2, (cyclo-CH2)(4), (cyclo-CH2)(5), (PhCH2)(2)) in toluene generate the six-coordinate high-spin (S = 5/2) [(Por)Fe(ONNR2)(2)]ClO4 compounds and a five-coordinate intermediate-spin (S = 3/2) [(OEP)Fe(ONNMe2)]ClO4 derivative in 57-72% yields (TPP = 5,10,15,20-tetraphenylporphyrinato dianion, TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion). The N-O and N N vibrations of the coordinated nitrosamine groups in [(por)Fe(ONNR2)(2)]ClO4 occur in the 1239-1271 cm(-1) range. Three of the six-coordinate [(por)Fe(ONNR2)(2)]ClO4 compounds and one five-coordinate [(OEP)Fe(ONNMe2)]ClO4 compound have been characterized by single crystal X-ray crystallography. All the nitrosamine ligands in these complexes bind to the ferric centers via a sole eta(1)-O binding mode. No arylnitrosamine adducts were obtained from the reactions of the precursor compounds [(por)Fe(THF)(2)]ClO4 with three arylnitrosamines (Ph2NNO, Ph(Me)NNO, Ph(Et)NNO). However, prolonged exposure of [(por)Fe(THF)(2)]ClO4 to these arylnitrosamines resulted in the formation of the known five-coordinate (por)Fe(NO) derivatives. The latter (por)Fe(NO) compounds were obtained more readily by the reactions of the three arylnitrosamines with the four-coordinate (por)Fell precursors.