Low-coordinated anions

The enhanced activity of the low-coordinated ions of the MgO surface has stimulated specific studies aimed at characterizing the nature of these centers [121-123]. We mentioned already that one important difference is the value of the MP at these sites and the consequent shift in position of the filled O 2p levels and stabilization of the empty Mg 3s levels. This results in a narrowing of the band gap in correspondence of these sites as found in Cl cluster studies [123] of the optical absorption energies of O ions at regular and low-coordinated sites of the MgO. A decrease of the transition energy from 7.1 eV for a terrace site to 5.7 and 4.5 eV for edge and corner sites, respectively, has been found [123]. 

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Low-coordinated anions ( 3.2). The O sites exhibit a completely different chemistry when located in the bulk, at the five-coordinated terraces, O sc, or at irregular sites with lower coordination, 0 4c and O sc, where the Madelung potential and the basicity of the site are different. 

Clearly, the presence of O species at the surface of MgO is not disconnected from the existence of other defects, low-coordinated anions or O vacancies. In fact, the complex interconversion of one center into another one is one of the reasons for the difficult identification of defect centers on oxide surfaces. 

For the specific case of isolated Pd atoms, a number of these sites have been investigated theoretically and a classification of the defects in terms of their adhesion properties is possible [32]. We first consider the case of Pd interacting with anion sites, Osc, O4C, or Osc. The binding energy of a Pd atom with these sites increases monotonically from f leV (Osc) to f.5eV (Osc) and consequently the distance of the Pd atoms from the surface decreases. This is connected to the tendency of low-coordinated anions on the MgO... 

When Pd is deposited on an MgO site like an ion at a terrace, a step, or a corner site the activation of C2H2 is much more efficient than for an isolated Pd atom. The structural distortion of adsorbed C2H2 follows the trend Pd-0 3c > Pd-0 4c > Pd-O sc >free Pd atom [215]. The donor capability of Pd increases as a function of the adsorption site on MgO in the order terrace < edge < corner. However, even on the low-coordinated anions the Pd atom is not an active catalyst for the cyclotrimerization. The increase of electron density on Pd due to the bonding with the MgO substrate at these irregular sites does not account for the observed reactivity. 

Even the cleanest of all the substrates shows areas where the periodic surface potential is perturbed. These sites can be generally called defects. Defects are generally classified into two main subclasses point defects, like corners, kinks, impurities or missing atoms and extended defects, like dislocations and steps. The type, concentration and characteristics of defects depend on several factors but the nature of the oxide and the history of the sample are no doubt the most important ones. In this section, two of the most commonly found MgO defects21,126 — low coordinated anion sites (steps and corners) and oxygen vacancies — will be considered with special emphasis on their interaction with metal atoms. 

Those originating from the reduction of low coordinated anion vacancies are designated here Fs a (H). Their spin centre interacts with the hydroxyl group of the chemisorption complex I (Fig.la). 

If the experimental conditions prior to recording the UV diffuse reflectance spectra are such that exclusively the saturation limited irreversible heterolytic H2 chemisorption (I) may occur, the intensity loss of the absorption band at 270 nm is small (Fig.6d) as compared to the pretreatment in terms of UV induced homolytic H2 splitting (Figs. 6b and c). This shows that the majority of the 3-coordinated O anions which exhibit a uniform UV excitation at 270 nm, are not able to split H2 heterolytically in the dark. For this purpose the specific site must — as discussed above — fulfill an additional requirement, namely, of being constituent of a low coordinated anion vacancy. 

C. Masalles, S. Borros, C. Vinas, F. Teixidor, Are low coordinating anions of interest as doping agents in organic conducting polymers , Advanced Materials 2000, 12, 1199. 

Pd-complexes described for the alternating copolymerization of olefins with carbon monoxide shows similar characteristics non- or low coordinating anions as well as two ligand sites occupied by nitril. The p-H-elimination is a relatively fast reaction... 

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