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. 

In the Blyholder model, CO interacts with the metal states through two different states, the 5a and 2jt orbitals, but due to their different symmetry they interact with different metal d orbitals, and the two interactions can be treated independently. Hammer, Morikawa, and Nprskov used the following expression to model the d contribution to the CO chemisorption energy ... 

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. 

For CO on Pt(lll) in acetonitrile containing 0.1 M NaCl04, the lifetime of the CO vibration increased from 1.5 to 2.1 ps with decreasing potential over a range of 2.6 V. These observations are consistent with the Blyholder model since, as the potential is adjusted to more negative values, electrons are expected to fill the 2 r orbital on CO, leading to a less efficient decay process [106]. 

Figure 10.7 Schematic representation of the orbital interaction scheme of CO with a surface metal atom according to the Blyholder model. Only interaction is considered of the 5a (donative) and 2 t orbitals (backdonative) of CO and d 2 and d atomic orbitals of the metal atom.

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