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, IR_P(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 (ν_CO^obs), is additive. The IR_P(CO) (i.e., the IR_P of carbon monoxide) is first defined as one-sixth that of the observed C≡O frequency (ν_CO^obs) of [Re(CO)₆]⁺. All subsequent IR_P(L) parameters of fac-[Re(CO)₃L₃] complexes are derived from IR_P(L) = ¹/₃[ν_CO^obs − 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 IR_P(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 (ν_CO^cal) is given by ν_CO^cal = S_R[∑IR_P(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 IR_P 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 IR_P(L) values should weaken the M−CO bond to a greater extent than ligands with low IR_P(L) values. The significance of the results and the limitations of the model are discussed.