Insertion coplanarity requirement

A general picture for the mechanism is shown in Scheme 4, which is based upon a theoretical analysis by Thom and Hoffmann. Here distinction between (2) and (2a) reflects the general assumption, supported by calculations, that the insertion step requires the M—H and C==C groups to be cis and coplanar, which need not be the case for the first-formed and/or thermodynamically most stable alkene complex (2). Thom and Hoffmann conclude that most or all metal hydrides will have some pathway that leads to hydrometallation without a large kinetic barrier, so long as none of the key intermediates along the way is too stable. The same inference was drawn for the bent metallocene systems discussed earlier (Figure 1) a kinetic barrier to insertion, found only for the cP-cases, is a consequence of the thermodynamic stabilization of alkene complex (2). ... 

The cis coplanar requirement predicted for insertions of metals has been confirmed by the observation that the following intramolecular reaction proceeds by olefin displacement of PPhs ... 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

Since the insertion of alkenes into M-C bonds proceeds via a four-center transition state, some requirements have to be accomplished namely (1) the alkene and the hydrocarbyl group have to take cis coordination sites (2) the double bond and the M-C bond have to become coplanar and (3) the 1,2-addition is cis, and this controls the relative stereochemistry at both carbons in case of prochiral alkenes (Scheme 6.33). The regio and stereoselectivity of this process controls the tacticity of polymers. 

---