Organometallic elementary steps

Mechanistically, the unusual reactivity of the starting tantalum hydrides [(sSiO)2Ta H] and [(=SiO)2Ta (H)3] towards ammonia to yield the amido imido complex [(=SiO)2Ta(=NH)(NH2)] can be fully rationalized in terms of classical molecular organometallic elementary steps. Scheme 2.21 offers an example of a such sequence of elementary steps with the noteworthy close analogy with methane activation by the same hydrides described above. 

Nature accomplishes many syntheses-even those of complex molecules-by sequences of elementary steps. In the last few decades, the blueprint of catalyzed cascade reactions has found fertile soil through the advent of transition metal catalysis in laboratories. Scrutinizing catalytic cycles and mechanistic insight has paved the way for designing new sequential transformations catalyzed by transition metal complexes in a consecutive or domino fashion. In particular, transition metal-catalyzed sequences considerably enhance structural complexity by multiple iterations of organometallic elementary steps. All this has fundamentally revolutionized synthetic strategies and conceptual thinking. 

Note that, while this reaction (as well as alkane hydrogenolysis) involves known elementary steps of molecular organometallic chemistry, it has not been discovered in homogeneous catalysis. One reason might be that highly... 

One should note that these mechanisms imply only well-known elementary steps of molecular organometallic chemistry [167]. 

Overall, this study shows that, like in molecular organometallic chemistry, the chemistry on metal surfaces follows similar elementary steps, and that it is possible to have a molecular understanding of catalytic phenomenon such as paraffin transformations on metal particles. 

Surface Organometallic Chemistry for Study of the Elementary Steps in Heterogeneous Catalysis... 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

From Organometallic Surface Chemistry to the Elementary Steps Occurring 13... 

From Organometallic Surface Chemistry to the Elementary Steps Occurring on Surfaces and Stabilization by the Surface of Rather Unstable Molecular Species... 

Heterogeneous catalysis is primarily a molecular phenomenon since chemical bonds are created and/or broken (between the molecule and the surface) this implies that surface organometallic fragments are intermediates in any catalytic reaction on a surface. If one can design and synthesize surface organometallic fragments and study their reachvity, especially elementary steps, then one possesses in principle a crihcal tool to better understand the mechanisms of heterogeneous catalysis. 

In stark contrast to the transport considerations, typical time-scales for elementary and non-elementary steps in organometallic chemistry and homogeneous catalysis range from vibrational motions of ca. 10 s or less [44], to ca. hours or even days for the formation of products and side products. Moreover, the three pri-... 

When we transfer the theoretical aspects of these models to or nometallic chemistry, we are aware of the fact that, e.g. when altering the number of rr-electrons by two (67T-, 4ir- or 2Tr-electrons) in the elementary steps of organometallic reactions, we have, step-by-step to await a change in the type of process (S or A) (in analogy to Fig. 1 in Scheme 2.1-3). On the other hand, we also can learn that the energies of the FMO s change systematically, going from 6n- to 4ir- to 27r-electron stems (from e.g. bis-ir-allyl- to rr-allyl-a-allyl to bis-o-allyl-ntetal-complexes). 

This section outlines the elementary steps in homogeneous catalysis using organometallic complexes, showing the role that these steps play in various catalytic cycles and industrial processes. 

Before discussing catalytic hydrogen mediated radical reactions, processes based on preformed organometallic reagents will be discussed, because in this manner the elementary steps can be highlighted in a more accessible manner. 

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