Agostic bonding, transition metal electronic

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. 

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. 

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. 

Abstract The agostic bond defines an intramolecular interaction where a a bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of a bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular, the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented. 

Several reviews on silane organometallic chemistry have been written, both on experimental [65-67] and theoretical aspects [68]. The agostic Si-H bond has been shown in early [69-88] and late [89-96] transition metal complexes in a wide variety of situations. The Si-H bond is a better candidate for an agostic interaction than C-H because it is more polarisable and the H is more hydridic. Computational studies confirm the presence of an M H-Si interaction in various systems. A recent review summarizes the computational studies up to 2002 [68]. It is interesting to include the o complexes. For instance the two hexacoordinated d complexes, 16 (characterized by X-ray [97]) and 17 (characterized by NMR [98]), probably have similar electronic M- Si-H interactions although no calculations have yet been carried out to test this. The difference between the two complexes could thus be mostly due to the fact that the Si-H bond is forced to remain in close vicinity to the metal in 16 because the alkene does not dissociate easily. This factor (essentially entropic) allows the experimental observation of weaker interactions. 

In 1983, Brookhart and Green [87] found a new bond called agostic in which hydrogen bonds with carbon and a transition metal simultaneously. Since this bond is a two-electron three-center bond, it is an electron-deficient bond. Both C—H and M—H bonds are longer by 5-20% than ordinary bonds and the reactivity is high... 

The insertion reaction, as its reverse, involves a metal-olefin-hydride species, and thus requires, in one direction or the other, a vacant site. It is thus necessary to start from a complex that has at most 16 valence electrons. The transition state then involves a square structure, then the last intermediate has a M-H-C agostic bond in equilibrium with the final complex. 

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