Bonding Blyholder 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). 

Figure 2.17. Schematic picture of the description of CO metal bonding via (top) the frontier-orbital picture, often denoted Blyholder model, with 5

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. 

Bonding of adsorbed CO to clean Fe was found to occur according to the Blyholder model (192), with complete dissociation at 300 K. On a sulfur-saturated (6 = 0.5) Fe surface, no stretching in the CO bond was observed, apparently due to reduction in forward- and back-bonding interactions. 

Two important points were missed in the Blyholder model, the first one is the role of the Pauli repulsion and the second is that, contrarily to what is assumed in this model, the a-donation also contributes to the red shift of CO chemisorbed on low index platinum surfaces.In fact, all bonding mechanisms, other than Pauli repulsion, but specially n-backdonation,... 

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 manifestation of increased polarizabihty for surface-coordinated CO is consistent with the Blyholder model [68] for CO adsorption. In the Blyholder model, the 5a orbital [the highest occupied molecular orbital (HOMO)j and the In orbital [the lowest unoccupied molecular orbital (LUMO)] of CO determine the surface chemical bond. Electrons are donated from the 5a orbital to the substrate and electrons from the substrate are back-donated to the 2n orbital of CO. The magnitude of n back-donation governs the bond strength of CO to the metal surface. 

The orbital interaction scheme in which the attractive contribution of the adsorbate surface bond is estimated from the donative and respective back-donative interactions is called the Blyholder model. It is the analogous to the Chatt-Dewar model which is used to describe chemical bonds in organometallic complexes. 

The Blyholder model [94-96] describes the MO interaction of a CO molecule at a transition metal surface. The lone pair of electrons on the carbon atom (5cr, HOMO) donates into the metal, forming a a -bond. The d-orbitals of the metal donate electron density into the anti-bonding (2jr, LUMO) orbital of CO giving rise to a jr-bond (back donation). The energetic shift of the 5cr orbital is therefore directly related to the strength of the bond formed between CO and the surface (lower 5cr... 

We shall start the theoretical exploration for a c(2 x 2)CO coverage on Ni(IOO) in very qualitative terms using the Blyholder model [20]. The 3highest occupied molecular orbitals (HOMO) of CO will form a bonding interaction with zYs/z hybrids on surface Ni atoms, 23.14. The Itt lowest unoccupied molecular orbitals (LUMOs) of CO form bonding combinations with xz/yz Ni surface orbitals, 23.15. 

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

Such shifts can be rationalized in terms of the model of synergic a and ir bonding of the CO ligand in metal carbonyls (29) as was proposed by Blyholder (30). The formation of a strong surface bond involves considerable back donation from metal d... 

The adsorption of carbon monoxide on metal surfaces can be qualitatively understood using a model originally formulated by Blyholder [45]. A simplified molecular orbital picture of the interaction of CO with a transition metal surface is given in Figure 6. The CO frontier orbitals 5a and 2n interact with the localized d metal states by splitting into bonding and antibonding hybridized metal-... 

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