Ligand Effect on Bonding in Gold(III) Carbonyl Complexes

We quantitatively assess the Dewar-Chatt-Duncanson (DCD) components of the Au(III)-CO bond and the charge density polarization at the CO, in a series of neutral, cationic, and dicationic bis-and monocyclometalated gold(III) complexes via charge-displacement (CD) analysis. A striking feature concerns the very small net electron charge flux from CO to the metal fragment which is unexpectedly stable toward both the charge of the complex and the oxidation state of gold (I, III). All systems exhibit a similar trend for the sigma charge rearrangement in the region of the carbonyl bond, where, by contrast, the Pi back-donation trend variation is large, which is strictly correlated to the change in CO bond distance and the shift in CO stretching frequencies, in close analogy with the gold(I) carbonyl complexes. In the whole series of gold(III) compounds, a large Au(III) <- CO sigma donation is measured (from 0.19 to 0.31 electrons), as well as a significant Au(III) -> CO Pi back-donation (from-0.09 to-0.22 electrons), which however is not generally able to completely balance the polarization of the CO Pi electrons in the direction from oxygen to carbon (C <- O) induced by the presence of the metal fragment [LAu(III)](0/+1/2). Surprisingly, all the gold(III) complexes in the series are characterized by a very small anisotropy in the Au(III) -> CO in-plane and out-of-plane Pi back-donation components, in sharp contrast with the marked anisotropy found before for the experimentally characterized [((CNC)-N-Lambda-C-Lambda)Au(III)CO](+) complex. A first attempt to figure out a rationale on the bonding/reactivity relationship for Au(III)-CO is made by performing a comparative study with an isostructural [((NNC)-N-Lambda-C-Lambda)Pt(II)CO](+) complex in a model water-gas shift (WGS) reaction.