Lewis bases agostic bonding

Alkenes and alkynes can also act as Lewis bases toward metals by using the two electrons in their 77 bonds. Such an interaction is called a it complex. The metal-alkene bond is a a bond due to its spherical symmetry around the axis formed by the metal and the midpoint of the C=C 77 bond. Even cr bonds such as the HU I and C-H bonds can act as two-electron donors to metals in this manner, and these compounds are called cr complexes. When a cr complex is formed in an intramolecular fashion, the bond is called an agostic bond or agostic interaction. Both 77 and cr complexes can be thought of as two-electron, three-center bonds in which three contributing orbitals comprise the occupied, bonding orbital. 

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. 

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