The reactions of (por)Os(CO) (por = TPP, TTP, OEP, TMP) with nitrosoarenes (ArNO; Ar = Ph, o-tol) in refluxing toluene generate the (por)Os(ArNO)(2) complexes in 45-76% yields (TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, TPP = 5,10,15,20-tetraphenylporphyrinato dianion, OEP 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion, TMP = 5,10,15,20-tetramesitylporphyrinato dianion). The nu(NO) of the coordinated PhNO groups in the (por)Os(PhNO)(2) complexes occur in the 1295-1276 cm(-1) range, and decrease slightly in the order TPP (1295 cm(-1)) > TTP (1291 cm(-1)) > OEP (1286 cm(-1)) > TMP (1276 cm(-1)). The reaction of (TTP)Os(CO) with 1 equiv of PhNO in CH2Cl2 at room temperature generates a 1:3 mixture of (TTP)Os(CO)(PhNO) and (TTP)Os(PhNO)(2) in ca. 40% isolated yield. The nu(CO) of (TTP)Os(CO)(PhNO) is at 1972 cm(-1) (KBr), which is 56 cm(-1) higher in energy than that of the precursor (TTP)Os(CO). When this mixture and excess PhNO are dissolved in toluene and the solution is heated to reflux, quantitative conversion to the (TTP)Os-(PhNO)(2) product occurs. LR monitoring of the reactions of (por)Os(CO) with 1 equiv of PhNO in CH2Cl2 reveal similar formations of the respective (por)Os(CO)(PhNO) intermediates for the TTP (1968 cm(-1); Delta nu(CO) = +74 cm(-1)), TMP (1966 cm(-1); Delta nu(CO) = +63 cm(-1)), and OEP (1958 cm(-1); Delta nu(CO) = +72 cm(-1)) analogues. Five of cm these (por)Os(ArNO)-containing complexes have been fully characterized by spectroscopic methods and by single-crystal X-ray crystallography. All the nitrosoarene Ligands in these complexes are attached to the formally Os-II centers via an eta(1)-N binding mode.