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

http://pubs.rsc.org/en/Content/ArticleLanding/2015/SC/C5SC02971F?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+rss%2FSC+%28RSC+-+Chem.+Sci.+latest+articles%29#!divAbstract


How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes
Giovanni Bistoni, Sergio Rampino, Nicola Scafuri, Gianluca Ciancaleoni, Daniele Zuccaccia, Leonardo Belpassi and Francesco Tarantelli

Chem. Sci., 2015, Accepted Manuscript
DOI: 10.1039/C5SC02971F
Received 11 Aug 2015, Accepted 23 Oct 2015
First published online 26 Oct 2015

The CO stretching response upon coordination to a metal M to form [(L)nM(CO)]m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π 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 σ donation and π back-donation charges, as well as of the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M–CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π 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 π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in "non-classical" carbonyls and we actually provide the framework in which the spectrosopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal-ligand moieties.