Oxygen-metal -bonding terminal

Figure 1.13 Raman spectra for a number of transition metal oxides supported on y-AI203 [75,102], Three distinct regions can be differentiated in these spectra, namely, the peaks around 1000 cm-1 assigned to the stretching frequency of terminal metal-oxygen double bonds, the features about 900 cm 1 corresponding to metal-oxygen stretches in tetrahedral coordination sites, and the low-frequency (<400 cm-1) range associated with oxygen-metal-oxygen deformation modes. Raman spectroscopy can clearly complement IR data for the characterization of solid catalysts. (Reproduced with permission from The American Chemical Society.)...

Many metal molybdates catalyse the reaction under consideration [4] and the active sites are widely believed to be associated with surface Mo atoms in octahedral coordination [5]. Octahedral coordination of Mo is only achieved in Fe-defective iron molybdates what is in accordance with the fact that maximum activity is obtained with catalysts with Mo/Fe atomic ratio greater than 1.5 [6]. The presence of two terminal oxygens double bonded to Mo in such coordination allows the methanol reacting molecules to be bonded simultaneously by two points. The activation of the hydrogen of the hydroxyl group produces methoxy species that are intermediates in the formaldehyde formation. The role of Fe in iron molybdates catalysts would be to act in the transfer of O2 and H2O between surface and gas phase [6] and to hinder the reduction of Mo [7]. 

Many attempts have been made to rationalize the structures and mechanisms of formation of polymetallates. Lipscomb observed that no individual MOg octahedral unit ever has more than two unshared, i.e. terminal oxygens (exceptions appear to be stable only in the solid state) and this has been explained on the basis of r-bonding between the metal and terminal oxygen atoms more than two of these... 

The early transition-metal-oxide clusters do not have metal-metal bonds, but the metals are linked by oxygen atoms that can also occupy non-bridging terminal positions. They are the heteropolyacids and their salts, the heteropolymetallates. These clnsters have been extensively developed in France by Sonchay s school now very active, in the United States by Pope and in Germany by Muller. - They have many kinds of structures, but the most known ones in oxidation catalysis are Keggin structures (XM i204o) in which the central atom X = Si, Ge, ... 

The latter parameter is a consequence of the ability of an element to involve low-energy, empty d orbitals in the metal-oxygen bond. Metal-oxygen tt bonds in which the oxygen loses almost entirely its properties as a base are therefore terminal bonds with no possibility of condensation to oligomers (polyoxoanions). This behavior is typical of transition metal oxides. For these elements, the size of the metal ion also limits the condensation the small size of Cr(Vl) forbids an increase in its coordination number. 

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