Chatt-Dewar-Duncanson picture

The chemical bonding of re coordinated ethylene is very similar to the Chatt-Dewar-Duncanson picture of CO coordination (Fig. 4.6). The donating orbital is the doubly occupied n orbital that is a-symmetric with respect to the normal to the surface. When adsorbed atop it interacts with the highly occupied dz2 surface atomic orbital and the partially filled s and pz orbitals. The ethylene LUMO is the empty asymmetric n orbital, which interacts with the surface dxz and px orbitals. The corresponding overall interaction is relatively weak and no hybridization on ethylene is assumed to occur. The orbital interaction diagram of the occupied ethylene n orbital with surface atom dz2 orbital is analogous to that sketched for the CO 5a orbital in Fig. 4.4. When this dz2 becomes nearly completely occupied, as occurs, for instance, for Pd or Pt, the ethylene-rc surface atom dz2 interaction... 

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. 

The nature of the metal-olefin bond was studied recently in our laboratory by analyzing the natural bond orbital (NBO) results (Huang, Padin, and Yang, 1999b). The main feature of the bonding can be seen from the population changes in the vacant outer-shell s orbital of the metal and those in the d shells of the metal upon adsorption. The NBO analysis, summarized in Tables X and XI, is generally in line with the traditional picture of Dewar (1951), and Chatt and Duncanson (1953) for metal-olefin complex-ation, i.e., it is dominated by the donation and back-donation contributions, as illustrated by Fig. 13. 

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 picture [38, 39]. hi this model, the interaction is viewed in terms of a donation of charge from the highest occupied 7i-orbital into the metal and a subsequent backdonation from filled metal-states into the lowest unoccupied 7i -orbital, see Fig. 12.4. Contrary to the case of the frontier orbital model for CO and adsorption, the DCD frontier-orbital model finds support from the experimental XES measurements [40]. 

Fig. 3. Schematic bonding pictures for transition metal atoms (M) binding to C2H4 via the Dewar-Chatt-Duncanson model.

The preceding pair of resonance descriptions is called the Dewar Chatt Duncanson model of tt bonding. Similar pictures can be drawn for metal-alkyne complexes, regardless of whether the second tt bond is acting as a tt donor, and for metal-1,3-diene complexes. 

The interaction between transition metal atoms (M) and fullerenes is often characterized in terms of the Dewar-Chatt-Duncanson model [67, 68], which implies electron donation from bonding orbitals at the organic ligand into unoccupied d orbitals of a symmetry at the metal as well as back donation from metal df-orbitals of 7T symmetry into vacant tt. Molecular orbitals at the interacting carbon-carbon double bond. We find that this picture describes the ) -coordinated Pd atom well at a (6-6 ) junction. The occupancy of the donating Pd -orbital remains essentially constant at 1.64 c as additional metal atoms are adsorbed. 

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