Lewis acids agostic bonding

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

The second pathway for electrophilic substitution requires coordination of the C—H bond to a Lewis acidic metal center, which depletes electron density of the C—H moiety and enhances its acidity and, hence, increases the propensity toward proton transfer. This reaction can occur with both aromatic substrates and alkanes. It is known that the coordination of dihydrogen can substantially increase the acidity of H2 4 and it is reasonable to assume that a similar effect can occur through metal coordination of C—H bonds, particularly with highly Lewis acidic metals. Although the pK values of hydrocarbons coordinated to metal centers have not been directly measured, the acidity of intramolecular agostic bonds has been reported.2... 

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

In 1983, Maurice Brookhardt and Malcolm Green published their concept of the intramolecular agostic C-H bond in complexes that feature a Lewis-acidic transition-metal center, i.e. those that possess a vacant coordination site. During the same year, Patricia Watson at the Du Pont company and John E. Bercaw at Caltech independently reported the activation of methane by d complexes of lanthanides and early transition metals (Lu and Sc respectively) according to the new a-bond metathesis mechanism (an intramolecular version being Schrock s a-elimination, in other d complexes, vide supra, which was developed by Tobbin Marks at Evanston with his seminal work in d° organoactinide chemistry also reported in 1982). 

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