Transition metals agostic bonding

Reviews on transition-metal-alkyl bond dissociation energies and the formation of C-H bonds by reductive elimination are also relevant to the mechanisms of catalytic processes. It has been proposed that [Ti(Me2PCH2CH2PMe2)(Et)Cl3] contains a direct-bonding interaction between the titanium atom and the j8-C-H system the evidence for such agostic C-H-M bonds in other systems has been reviewed. Obviously, such bonds have important implications in alkane activation and it is particularly exciting that kinetic studies of the exchange reaction shown in equation (1) indicate that a bimolecular pathway predominates. ... 

In these compounds the Si-H bcnid is likely to act as a o-donor to the transition metal (agostic interaction), or the Si-bound hydrogen atoms act as o-donors towards the transition metal, thus constraining the Si atom in close proximity to the transition metal even without o-donor action from the latter to the Si atom. Besides, there are review articles on this topic, as mentioned in the introduction [36-40]. Hence, compounds such as 158 and 159 will not be discussed in detail here [327, 328]. Also, the same applies to compounds with Si transition metal bond which only achieve Si-hypercoordination by the presence of metal-bonded hydrogen atoms in closer proximity of the Si atom, while the rest of the Si coordination sphere is almost tetrahedral, thus less indicative of H Si donor action (e.g., 160) [329],... 

The characteristic tendency of hypovalent transition metals to interact in a side-on (T-shaped) manner with nearby alkyl C—H bonds has been designated as the agostic effect.45 Agostic M- C—H interactions can also be identified with general 3c/2e donor-acceptor interactions of 2c— lc type. Availability of suitable lc acceptor orbitals is a signature of hypovalent early transition metals. 

The alkyne-to-vinylidene tautomerization processes on various transition metal centers have also been discussed. Three different pathways for the formation of vinylidene from p -acetylene on electron-rich transition metals were the most theoretically studied. Most studies suggested that the favorable pathway proceeded via an intermediate with an agostic interaction between the metal center and one C—H bond followed by a 1,2 hydrogen shift (the bl+b2 pathway shown in Scheme 4.5). The reverse process, the vinylidene-to-p -acetylene tautomerization, was also discussed. It was found that complexes with electron-poor metal centers were able to mediate the reverse process. 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

Two molecules can be combined to form an ion-pair through a a coordination bond, in which one molecule provides its X-H (X = B, C, N, O, Si) a bonding electrons to a transition metal atom (such as Zr) of another molecule. A good example is [(C5Me5)2Zr+Me][B Me(C6F5)3], whose structure is shown in Fig. 11.5.5. This bonding type is called an intermolecular pseudo-agostic (IPA) interaction. 

Figure 11.5.7 shows the molecular structures of some transition metal complexes in which agostic bonding has been characterized by either X-ray or neutron diffraction. Structural data for these compounds are listed in Table 11.5.2. 

The recent discovery of many metal complexes having t/2-H2 ligands or agostic C—H bonds has focused attention on a type of bonding that had hitherto seemed to be a domain of early main group elements and metal clusters three-center, two-electron bonds. As more and more examples are found, it has become obvious that such bonds are probably more widespread among transition metal compounds than had been commonly assumed and that it is about time to recognize the features the various species have in common. 

The basic molecular orbital scheme shown in Figure 2.22 for the coordination of a H-H o-bond to a transition metal is in principle applicable to the coordination of any H-element o-bond. In particular, there are many examples of 3c-2e bonding involving coordination of C-H o-bonds (Figure 2.24) and other heteroatoms, e.g. B-H, Si-H, Ge-H, N-H bonds (Figure 2.25). The term agostic has been used to denote such interactions, in particular for situations involving C-H-M interactions, and these will be met subsequently. 

There is a vast and rapidly increasing literature on transition metal hydrides, their chemical reactions and their role in hydrogen transfers, as in catalytic reactions of alkenes and other substrates. We can only outline the present knowledge which includes also what are called agostic H atoms and new types of H-bonding as well as compounds with M—H and M(H2) bonds. 

---