Parametrization of the contribution of mono- and bidentate ligands on the symmetric C=O stretching frequency of fac-[Re(CO)3]+ complexes

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 d6 fac-[Re(CO)3L3] 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≡O stretching frequency (νCOobs), 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 (νCOobs) of [Re(CO)6]+. All subsequent IRP(L) parameters of fac-[Re(CO)3L3] complexes are derived from IRP(L) = 1/3[νCOobs − 3IRP(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 d6 fac-[M(CO)3L3] complexes (M = Mn, 99Tc, and Ru) and cis-[Re(CO)2L4]+ species and by calculations at the density functional level of theory. The predicted symmetric C≡O stretching frequency (νCOcal) is given by νCOcal = SR[∑IRP(L)] + IR, where SR and IR 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 EL 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