Y agostic interaction

The y-agostic interaction is such that the electron pair in the C, -He bond of the alkyl substituent at the metal is partly donated to the electron-poor metal this may play an important role in making more simple the insertion of a coordinated olefin into the metal-alkyl bond. This hypothesis has been tested in some instances [346-350], showing metal-hydrogen interaction via an oc-agostic bond. 

The activation barriers for the insertion starting from 14c are visibly lowered compared to those obtained for 14. (Table 4-1). Decrease in the barriers demonstrates a role of the y-agostic interactions in stabilization of the TS for the olefin insertion reactions. However, for both metals the barriers are still substantially higher than the activation barriers for olefin insertion observed in the homopolymerization processes. 

Figure 6.4 Alkene insertion with a- and y-agostic interactions in the transition state 6.12 and intermediate 6.13. Such interactions are important for stereoregular polymerization by metallocene catalysts.

Cationic alkyl complexes Cp 2Zr(CH2CH2R)(THF)" contain normal undistorted alkyl groups as established by NMR and IR data. Reaction with PMcj yields free THF and the corresponding PMe complexes Cp 2Zr(CH2CH2R)(PMe3)+ [R = H (6), Et (7), Ph (8), CMej (9)] which exhibit strong y -agostic interactions [Eq. (19) (71,72). X-ray analysis of the... 

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