Ji backbonding

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

The use of TEP is also not without some drawbacks. The weakening of the CO bond via back-donation into the r co antibonding orbital is dependent on the ability of the metal center to undergo d —+ it co backbonding while this is typically correlated to the electron density of the metal center, it also depends somewhat on the Ji-acceptor nature of the NHC ligand. Nolan, Jacobsen, and Cavallo... 

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