Diethylnitrosamine reacts with [(TPP)Fe(THF)(2)]ClO4 (TPP = 5,10,15,20-tetraphenylporphyrinato dianion) in toluene to generate the bis-nitrosamine complex, [(TPP)Fe(Et2NNO)(2)]ClO4, in 96% isolated yield. The related [(TTP)Fe(Et2NNo)(2)]SbF6 (TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion) complex is prepared in 70% isolated yield via a similar reaction in CH2Cl2. Reaction of [(TPP)Fe(Et2NNO)(2)]ClO4 in CH2Cl2 with NO gas results in the displacement of one of the Et2NNO ligands to give the air-sensitive and thermally sensitive [(TPP)Fe(NO)-(E(t)2NNO)]ClO4 derivative. Reaction of (OEP)Ru(CO) (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion) with NOBF4 in CH2Cl2 gives [(OEP)Ru(NO)(H2O)]BF4 as the final isolated product (after exposure to air) in 71% isolated yield. The aqua ligand is then displaced by Et2NNO in CH2Cl2 to give [(OEP)Ru(NO)(Et2NNO)-BF4 in 82% isolated yield. The valence isoelectronic (OEP)Ru(CO)(Et2NNO) compound is prepared in 71% isolated yield by the addition of excess Et2NNO to (OEP)Ru(CO) in CH2Cl2. The nitrosyl amine complex [(OEP)Ru(NO)(H2O)]BF4 is prepared (i) in 78% yield by diethylamine addition to [(OEP)Ru(NO)(H2O)]BF4 or (ii) in 71% isolated yield by diethylamine addition to [(OEP)Ru(NO)(Et2NNO)]BF4. The osmium nitrosamine complexes, (TTP)Os(CO)(Et2NNO) and (OEP)Os(CO)(Et2NNO), are prepared in 74% and 66% yields, respectively, by diethylnitrosamine addition to the precursor (porphyrin)Os(CO) compounds in CH2Cl2. The nitrosyl [(OEP)Os(NO)(Et2NNO)]BF4 derivative is prepared in quantitative yield (by IR and H-1 NMR spectroscopy) by the reaction of (OEP)Os(CO)(Et2NNO) with NOBF4. Labeling studies using (NOBF4)-N-15, (Et2NNO)-N-15, and (Et2NNO)-O-18 have been used to assign the nitrosyl and nitrosamine bands in the IR spectra of several of the complexes. The solid-state structures of [(TPP)Fe(THF)(2)]ClO4, [(TPP)Fe(Et2NNO)(2)]ClO4, [(OEP)Ru(NO)(H2O)]BF4, (OEP)Ru(CO)(Et-2-NNO), and (TTP)Os(CO)(Et2NNO) have also been determined by single-crystal X-ray diffraction. The Et2NNO ligands display a rare eta(1)-O binding mode in all three nitrosamine complexes.