DOI: 10.1002/chem.201000608 On the Dewar–Chatt–Duncanson Model for Catalytic Gold(I) Complexes Nicola Salvi, Leonardo Belpassi, and Francesco Tarantelli* [a] Introduction In recent years, homogeneous gold catalysis has been estab- lished as a powerful and versatile tool in chemists hands. [1–6] The binding of Au I complexes to alkenes and, especially, al- kynes enables the chemo-, regio-, and stereoselective activa- tion of unsaturated carbon  carbon bonds, often more effec- tively than other transition metals. [7–12] A growing body of theoretical data exists on Au I chemistry, [13–15] but an under- standing of the nature of the chemical bond between Au I and the reacting substrate in the catalytic intermediates and of its reactivity is still limited and lacking predictive capaci- ty. [16] One significant, still unsettled, and much debated issue concerns the interpretation of the electronic structures of these species in terms of the well-known Dewar–Chatt– Duncanson (DCD) model, [17–20] which, ironically, was first in- troduced almost 60 years ago precisely to describe the h 2 co- ordination of ethene to a coinage-metal atom. In short, the debate concerns the extent and relative importance of sub- strate-to-metal (S !M) donation and metal-to-substrate (M!S) back-donation. Experimental clues are only indirect. Remarkably, for ex- ample, Au I carbonyls are “nonclassical” in that they display a blueshift of the CO stretching frequency, [21] which is com- monly associated with a lack of back-donation. [11, 22, 23] In the case of Au I –alkynes or Au I –alkenes, relevant unambiguous experimental evidence is more difficult to obtain due to the fact that both the S !M donation and M!S back-donation components tend to weaken the C  C bond, [24–26] but recently reported data were also deemed consistent with the lack, or marginal role, of back-donation. [27] The lack of back-dona- tion may have an impact on the peculiar and powerful cata- lytic properties of Au I because, quite simply, it should tend to increase the electrophilicity of the coordinated sub- strate. [11] Most of the available theoretical work involving DCD- type analyses of alkene and alkyne complexes of Au I deals with bare Au–ethene +[25, 28–32] or Au–ethyne +[30] and general- ly tends to suggest that s donation largely dominates over p back-donation. Only when the presence of ancillary ligands (F  , bipyridines) was studied by natural bond order (NBO) analysis of orbital populations was a significantly larger or even dominating p back-donation found. [25, 31] However, the most recent investigations carried out on realistic Au I – alkene [33] and Au I –alkyne [26, 34–36] catalytic intermediates using NBO orbital interaction energies all support the view of a dominating donation component. Rather than on the basis of energy decompositions, a con- clusive assessment of coordination bonds in terms of the DCD model requires a clear-cut and noncontroversial defi- nition of the DCD components, donation and back-dona- tion, which can only emerge as a result of a detailed analysis of the changes in electron density taking place upon forma- Abstract: We provide a rigorous model-free definition and a detailed theoretical analysis of the electron- charge displacements making up the donation and back-donation compo- nents of the Dewar–Chatt–Duncanson model in some realistic catalytic inter- mediates of formula L–Au I –S in which L is an N-heterocyclic carbene or Cl  and S is an h 2 -coordinated substrate containing a C  C multiple bond. We thus show, contrary to a widely held view, that the gold  substrate bond is characterized by a large p back-dona- tion component that is comparable to, and often as large as, the s donation. The back-donation is found to be a highly tunable bond component and we analyze its relationship with the nature of the auxiliary ligand L and with structural (interdependent) factors such as metal–substrate bond lengths and carbon pyramidalization. Keywords: coordination modes · density functional calculations · electronic structure · gold · homo- geneous catalysis [a] N. Salvi, L. Belpassi, F. Tarantelli Dipartimento di Chimica, Università di Perugia and I.S.T.M.-C.N.R. 06123 Perugia (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000608. Chem. Eur. J. 2010, 16, 7231 – 7240  2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 7231 FULL PAPER