Chemical Dewar-Chatt-Duncanson model

The electronic structure of the surface chemical bond is discussed in depth in the present chapter for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated -systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone pair interactions, and (v) saturated hydrocarbons (physisorption). 

Powder as well as MAS studies are available for several r/ -bonded olefins. The spans of the chemical-shift tensors are reduced with respect to the free olefins, which is discussed in terms of the Dewar-Chatt-Duncanson model of fx-donation and tt- back-bonding." " " The bond lengths and the orientations of the shielding tensor elements are available from dipolar-chemical shift methods and 2-D spin-echo experiments on the doubly labelled oleflns." " 77 -cyclopentadienyl and 77 -benzene ligands of transition-metal complexes, but also some derivatives of alkali or main-group elements," exhibit single resonances and shielding tensors of axial symmetry at room temperature. Both observations point to relatively fast rotations around the respective 5- and 6-fold local rotor axis. ... 

Like in the case of olefin complexes, the chemical shift t of the coordinated CHR group increases by 2-3 ppm compared to the free allene. This is also in agreement with the Dewar-Chatt-Duncanson model of the metal-allene bond (rehybridization sp -> sp ) according to the valence bond theory. 

Figure la shows a schematic representation of the Dewar-Chatt-Duncanson (DCD) model. The pivotal idea is that the olefin serves as a donor and an acceptor at the same time. There is ligand metal donation and metal -> ligand back-donation. The former interaction involves a donor orbital of the ligand which has n symmetry in the free ligand but cr symmetry in the complex. The metal acceptor orbital is mainly the d 2 orbital of the metal. Quantum chemical calculations have shown that the valence s orbital of the metal is less important as an acceptor orbital than the d 2 orbital. The metal ligand back-donation takes place via a d( r) orbital of the metal and the n orbital of the olefin. 

The description of the bonding of unsaturated hydrocarbons to metals was originally developed by Dewar, Chatt and Duncanson and is now known as the well-established DCD model based on a frontier-orbital concept [82]. In this model, the interaction is viewed in terms of a donation of charge from the highest occupied -orbital into the metal and a subsequent backdonation from filled metal-states into the lowest unoccupied -orbital, see Figure 2.33. Contrary to the case of the standard Blyholder model for CO and N2 the DCD frontier-orbital model is supported by experimental XES measurements [83]. In the present section, we will show how we can experimentally identify and quantify the contributions of the different -orbitals involved in the interaction with the surface. The DCD model will be shown to very well describe the chemical bonding of ethylene on Cu and Ni surfaces. Furthermore, the differences in bonding of benzene to Cu and Ni will be discussed. 

The partial confusion arising after Dewar s and Chatt s reviews were published, was resolved after Chatt and Duncanson reported in 1953 in the Journal of the Chemical Society the results of infrared spectroscopic studies on a range of olefin platinum(II) complexes [38]. In this highly cited paper they proposed, with particular reference to Dewar s model, that in the olefin platinum(II) complexes the cr-type bond would be formed by overlap of the filled re-orbital of the olefin with a vacant 5d6s6p2 hybrid orbital of the platinum atom, and the re-type bond by overlap of a filled 5d6p hybrid orbital of the metal with the empty antibonding re-orbital of the olefin (Fig. 7.8). In addition, Chatt and Duncanson illustrated how the model could be used to interpret not only the physical properties of the olefin platinum compounds, such as the spectroscopic data and dipole moments, but also their reactivity and their greater stability compared to the olefin silver salts. 

The orbital interaction scheme that describes bonding of ligands as CO, but also of other ligands, in terms of the sum of donative and backdonative interactions is called the Chatt-Dewar-Duncanson picture of the chemical bond. Historically it was first used to describe bonding of ethylene. As we will see later, bonding to surfaces (Section 4.4.1.2) is quite analogous and there it is called the Blyholder model. 

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