Reactivity, inorganic oxides towards

The amounts of NpH or HCl released in the gas phase indicate that the reaction of (1) and (2) with the surface of inorganic oxides such as Si02 and Nb205 leads to a mixture of surface complexes. The use of different chemical reactions for characterization revealed that the high catalytic activity is not correlated only to high concentration of neopentylidene W surface complexes. In the case of (1), reduced species (W or less) seem to be involved these could result from reductive elimination of dineopentyl (indeed detected during the impregnation step). When such reduction is not possible, as it is the case for (2)/ support, activity seems to be correlated to a neopentylidene surface complex, whose reactivity towards olefins is apparently dependent on electronic factors. 

The sol gel chemistry of sihcon alkoxides is much simpler (see Silicon Inorganic Chemistry)P Si is fourfold coordinated (N = z = 4,) in the precursor as well as in the oxide so that coordination expansion does not occur. The electronegativity of Si is rather high compared to transition metals. Silicon alkoxides are therefore not very sensitive toward hydrolysis. Their reactivity decreases when the size of the alkoxy groups increases. This is mainly due to steric hindrance, which prevents the formation of hypervalent sihcon intermediates (see Hypervalent Compounds). Silicon alkoxides, Si(OR)4, are always monomeric. Heterometallic alkoxides have never been obtained via the reaction of a sihcon alkoxide with another alkoxide. Silicon alkoxides have to be prehydrolyzed before Si T M bonds can be formed. 

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