On the Dewar-Chatt-Duncanson model for Au(I) complexes with Nitrogen Heterocyclic Carbenes(NHC)

The analysis of the electron density changes occurring upon formation of a bond can be studied through the charge-displacement (CD) function: where ??(x,y,z') is the difference between the electron density of a complex and that of its non-interacting fragments, placed in the same position they occupy in the complex. This function was successfully introduced by some of us to study the chemical bond between gold and the noble gases [1]. It defines, at each point z along a chosen axis, the amount of electron charge that, upon formation of the bond between the fragments, moves across a plane perpendicular to the axis through z. A simple extention of the CD function permits the unambiguous separation of the components of the Dewar-Chatt-Duncanson bond model and, most important, a detailed analysis of their spatial distribution over the whole molecular region[2]. Here, we applied[3] this new approach, in combination to the theoretical determination of the chemical shift tensor of the carbenic carbon, in a larger set of charged and neutral complexes (NHC-Au-L). We found: 1) a rather good linear correlation between the chemical shift of the carbenic carbon and the donation component of CD (figure) and 2) that the ?-backdonation component must be taken into account in order to correctly describe the gold-carbene bond since its contribution may approach up to 50% of the ?-donation. An analysis of CD and of the chemical shift tensor casts light on these findings.