Metal Pauli repulsion

The nature of bonding of the adsorbed species to the model cluster of metal surfaces can be analyzed in terms of the so-called constrained space orbital variation (CSOV) method. For halogen anions adsorbed on various silver surfaces, it has been found that Pauli repulsion, metal polarization, and charge transfer to the metal surface mainly contribute to the binding energy of the ions [104, 301]. 

The bonding of ions to metals is dominated by Coulomb attraction since there is a significant difference in electron affinity between the metals and ions. The bonding also involves a redistribution of charge through intermolecular charge transfer (between adsorbed ions and the surface) and intramolecular polarization (in ions and on the surface), which reduces the Pauli repulsion. 

Transition metal complexes comprise a typical example. The lone pair orbitals of ligands like CO, H20, Cl-, and O2- experience significant repulsion from the upper core shells 3s and 3p (first transition series). The Pauli repulsion with these shells determines the repulsive wall in the metal-ligand -versus-R curve. Of course, there is also overlap with deeper core orbitals, and so their effect is less important. An illustration is provided elsewhere (Figures 2 and 3 of Ref. 35), where the behavior of the Pauli repulsion is demonstrated along the Mn—O bond distance in MnO. The fragments are Mn2+, which has 5 electrons with spin up in the 3d orbitals, and the 02 cage, which has 5 electrons... 

M. A. Buijse and E. J. Baerends,/. Chem. Phys., 93,4129 (1990). Analysis of Nondynamical Correlation in the Metal-Ligand Bond. Pauli Repulsion and Orbital Localization in MnOj. 

Figure 4 shows that contrarily to what it has been proposed for metal overlayers, the CO (2n) backdonation contribution does not permit to explain the experimental, and also calculated, linear relationship between the interaction energy and the Pd core level shift, pointing out to a different chemical interpretation of the phenomenon in alloys, at least PdCu alloys, and overlayers. Finally, the CSOV and projection analyses permit to explain the C-0 stretch insensitivity to alloy composition. Since charge-transfer processes and Pauli repulsion do not vary significantly with copper content, only the correlation contribution is expected to influence this observable, but... 

This leads to a stronger Pauli repulsion with the CO molecule the Pd-C bond distance is in fact 0.10-0.15 A longer than for low-coordinated 0 sites. The augmented electron density on the metal, however, reinforces also the back donation mechanism so that the CO coq for CO/Pd/F c is similar as for the low-coordinated O sites despite the longer Pd-C distance. Table 2. Thus, while the computed CO adsorption energies, 0.3-0.5 eV, are consistent, although somewhat underestimated, with the experimental TDS value,... 

From a simple frontier orbital point of view the primary features of H—H bond activation are (1) attractive metal cluster donor interactions between the high-lying unoccupied H2 a antibonding orbital and the highest occupied molecular orbital M (HOMO) of the cluster, (2) attractive metal acceptor interactions between the occupied Hj ff bonding orbital and the lowest unoccupied cluster orbital M(LUMO), and (3) Pauli repulsion between the filled Hj ff orbital and filled cluster valence orbitals. 

The results show that nanoclusters adsorbed on the regular MgO(OOl) surface do not necessarily tend to adhere to the surface with the largest possible number of metal atoms (surface wetting) but rather that they keep some bond directionality. This results from the balance of various terms, the energy gain due to the bond formation with the O anions, the Pauli repulsion with the surface, and the loss of metal-metal bonding within the cluster due to distortions of the metal frame. 

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