Experimental evidence including infrared spectra for the formation of the dinitrosyl metalloporphyrin complexes M(P)(NO)(2) (M = Ru or Fe, P = tetraphenylporphyrin (TPP), octaethylporphyrin (OEP), or tetra-m-tolylporphryin (TmTP)) is described. Although observation of a single NO stretching band in the IR spectrum of each M(P)(NO)(2) complex first suggested a centrosymmetric (D-4h or C-2h) structure, DFT geometry optimizations and frequency calculations of model complexes indicate that the trans-syn (C-2v) conformation should be more stable. The frequency calculations resolve the apparent ambiguity in the IR spectra in terms of the relative oscillator strengths of the predicted IR bands.