When the Tolman Electronic Parameter Fails: A Comparative DFT and Charge Displacement Study of [(L)Ni(CO)(3)](0/-) and [(L)Au(CO)(0/+)

In this study we have examined 42 [(L)M(CO)(n)](+/-/0) complexes (M = Ni and Au), including neutral ligands, such as phosphines and carbenes, and anionic ones. For each complex, the carbonyl stretching frequency (nu(CO)) and the amount of charge donated from the ligand to the metal (CT) have been computed on the basis of DFT calculations. For nickel complexes, the two observables nicely correlate with each other, as expected from the theory underlying the Tolman electronic parameter. On the contrary, for gold complexes a more complex pattern can be observed, with an apparent differentiation between phosphine ligands and carbon-based ones. Such differences have been explained analyzing the AuL bond in terms of Dewar-Chatt-Duncanson bonding constituents (sigma donation and pi back-donation). Our analysis demonstrates that in linear gold(I) complexes, nu(CO) depends only on the metal-to-ligand pi back-donation.