The temperature dependence of the Sorer band of various metal-octaethylporphyrins [i.e., Ni(II), Cu(II), and Pd(II)] dissolved in the glass-forming mixture 50% (v/v) isopentane/ethyl ether in the temperature range 300-40 K was studied. Co(II)-octaethylporphyrin dissolved in dichloromethane was also investigated in the temperature range 300-180 K. The aim of the work was to investigate the role of the central metal in the conformational flexibility of porphyrins and in communicating solvent motions to their macrocycle. We used resonance Raman spectroscopy to determine the vibronic coupling of high-frequency modes to the electronic transition into the porphyrin B state. The corresponding coupling of a "bath" of low-frequency porphyrin/solvent motions was taken into account by a temperature-dependent Gaussian width (sigma) of the Soret band. Following the approach of Melchers et al. (Biophys. J. 1996, 70, 2092-2099), the mean square fluctuations (MSF) of the central metal atom with respect to the porphyrin plane were determined from the sigma temperature dependence. MSF values exhibited a harmonic behavior only at low temperatures, whereas an increase of the MSF well above the predictions of the harmonic model was observed above the glass transition temperature of the solvent mixture. This result was rationalized by invoking coupling of solvent motions to the vibrations of the central metal atom. The magnitude of this coupling is strongly metal dependent in that it is large for Ni(II) and Pd(II), weak for Co(II), and almost negligible for Cu(II). This dependence is discussed in terms of the ionic radii and electronic structure of the various metals. Moreover, for Ni(II), we found the MSF to have an amplitude comparable with what has earlier been obtained for iron-heme complexes in proteins and in solution.