How pi back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes

The CO stretching response upon coordination to a metal M to form [(L)(n)M(CO)](m) complexes (L is an auxiliary ligand) is investigated in relation to the sigma donation and pi back-donation components of the M-CO bond and to the electrostatic effect exerted by the ligand-metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)](0/+) gold(I) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of s donation and pi back-donation charges, as well as to quantify the sigma and pi components of CO polarization. It is found that all complexes feature a comparable charge flow of sigma symmetry (both in the M-CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, pi back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C -> O or C <- O) of the CO pi polarization. As a result, we conclusively show that pi back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the pi donor properties of metal-ligand moieties.