Model Studies - Structures and Reactions

Sometimes it is not the catalyst metal itself which is used but a neighbouring metal with similar properties. For example, the methylrhodium complex [Rh(Me)(CO)2l3] was widely accepted as an intermediate in the carbonylation of MeOH to AcOH involving [Rh(CO)2l2] and Mel even though it had never been observed. This was in part because the corresponding Ir complex was stable and well characterized. The rhodium species was subsequently observed by carrying out the reaction in a non polar solvent at a very high [Mel] which increased the rate of its formation and slowed down further reaction (see Section 4.2.4). 

For heterogeneous catalysts it is more difficult to study the metal sites and their interaction with reactants under catalytic conditions. Many solid catalysts consist of metal crystallites or oxide particles in a variety of sizes and forms which themselves contain a number of different environments for the metal. Much of the routine characterization of these catalysts takes place before and after, rather than during, the catalytic reaction. Other studies probing surface bound species may have to be carried out under high vacuum rather than under the typical working conditions of the catalyst (see Appendix B for a summary of some relevant aspects of surface science). 

Thus even if we do not observe the intermediate species in a metal catalysed reaction under working conditions, we can make good models or manipulate the conditions to enable us to observe these intermediates, or in some way modify the metal complexes to make the structural types we believe are important. 

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