Dihydrogen transition metal complexes

Why are transition metals so well suited for catalysis A complete treatment of this critical question lies well beyond the scope of this book, but we can focus on selected aspects of bond activation and reactivity for dihydrogen and alkene bonds as important special cases. Before discussing specific examples that involve formal metal acidity or hypovalency, it is convenient to sketch a more general localized donor-acceptor overview of catalytic interactions in transition-metal complexes involving dihydrogen49 (this section) and alkenes (Section 4.7.4). 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

THE REACTIVITY OF TRANSITION METAL COMPLEXES WITH DIHYDROGEN... 

As shown in Figure 53, the interaction of dihydrogen with the metal centre of transition metal complexes can be described as a two-electron... 

In the reactions above we have not explicitly touched upon the reactions of dihydrogen and transition metal complexes. Here the reactions that involve the activation of dihydrogen will be summarised, because they are very common in homogeneous catalysis and because a comparison of the various mechanisms involved may be useful. Three reactions are usually distinguished for hydrogen ... 

Do the bonding principles found for dihydrogen-transition metal complexes apply to main group cations ... 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Stabilization of transition states by intramolecular dihydrogen bonding explains the high degree of fluxionality of polyhydride transition metal complexes. 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

Electron densiity, in lead triacetate arylations, 9, 397 Electron donor number, transition metal complex ligands and unchanging coordination number, 1, 12 and variable coordination number, 1, 12 Electron energy loss spectroscopy, dihydrogen binding on surfaces, 1, 682 Electronic effects... 

Dihydrogen is evolved prior to reduction of dinitrogen, as exemplified by transition metal complexes [Eqs. (86) and (87)]. 

The coordination of a dihydrogen molecule (H2) to a transition metal complex in W(CO) i(P Pr3)2(H2) (Fig. 1), was discovered about 20 years ago (1,2) and was the first well-established a-complex. In this complex the H2 ligand is nearly intact with a stretched H-H bond (H-H distance is 0.89 vs. 0.75 A in free H2) while dihydrides have H-H > 1.6 A. 

It is now believed that dihydrogen initially adds to many transition metal complexes in the form of a hydrogen atoms is broken upon addition to the transition metal complex and homogeneous hydrogenation catalysts can equilibrate mixtures of ortho- and parahydrogen. 

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