A ligand parameter, IRP(L), is introduced in order to evaluate the effect that different monodentate and bidentate ligands have on the symmetric C O stretching frequency of octahedral d(6) fac-[Re(CO)(3)L-3] complexes (L = mono- or bidentate ligand). The parameter is empirically derived by assuming that the electronic effect, or contribution, that any given ligand L will add to the fac-[ReCO3](+) core, in terms of the total observed energy of symmetric C 0 stretching frequency (nu(COobs)) is additive. The IRP(CO) (i.e., the IRp of carbon monoxide) is first defined as one-sixth that of the observed C O frequency (nu(COobs)) of [Re(CO)(6)](+). All subsequent IRp(L) parameters of fac-[Re(CO)(3)L-3] complexes are derived from IRp(L) = 1/3[nu(COobs) - 3IR(P)(CO)]. The symmetric C O stretching frequency was selected for analysis by assuming that it alone describes the "average electronic environment" in the IR spectra of the complexes. The IRp(L) values for over 150 ligands are listed, and the validity of the model is tested against other octahedral d(6) fac-[M(CO)(3)L-3] complexes (M = Mn, Tc-99, and Ru) and cis-[Re(CO)(2)L-4](+) species and by calculations at the density functional level of theory. The predicted symmetric C O stretching frequency (nu(COcal)) is given by nu(COcal) = S-R[Sigma IRP(L)] + I-R, where S-R and I-R are constants that depend upon the metal, its oxidation state, and the number of CO ligands in its primary coordination sphere. A linear relationship between IRp values and the well-established ligand electrochemical parameter E-L is found. From a purely thermodynamic point of view, it is suggested that ligands with high IRp(L) values should weaken the M-CO bond to a greater extent than ligands with low IRP(L) values. The significance of the results and the limitations of the model are discussed.