Metal oxides poly bonding

For more than sixty years, the basic approach to the crystal chemistry of ionic structures, particularly those of the solid metal oxides, has placed almost exclusive emphasis on the geometry of the anion array. Consideration of the cations is normally limited to a discussion of their immediate environment, i.e. to bond lengths and coordination poly-hedra. 

Figure 6.13 shows the adsorption isotherms for PAM on poly Si and Si02 as a function of PAM concentration. Adsorption of PAM on oxide surfaces increases and reaches a plateau level of approximately 0.23 mg/m. However, PAM is scarcely adsorbed on poly Si surfaces. This is driven by the difference in hydrophobicity, which affects the interaction between PAM and each surface. At high pH, the negative site MO of metal oxide (MO) surface bonds with the weakly acidic NH2 function. Therefore, the interaction between SiO of Si02 surface and NH2 group of PAM led to the selective adsorption of PAM on Si02. 

As mentioned above, the classical polyoxoanion forming metals are molybdenum and tungsten in the oxidation state VI. It is assiuned that for these metal cations the combination of ion-radius and -charge and the availability of empty d-orbitals for the formation of metal-oxygen-rx-bonds is especially favorable. However, other metals can act as polyoxoanion-builders as well. Vanadimn-, niobiiun- and tantalum-V, technetium-, rhenium-, ruthenium- and osmium-Vl, chromium-, molybdenum-, timgsten-, technetium- and rhenium-V and tita-niiun-, vanadium-, chromium-, molybdenum- and tungsten-IV can build poly-oxometallate-clusters. 

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