Metallocenes hydrometallation

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). ... 

A quite different case is presented by the bent metallocene systems, [Cp2MH(alkene)] where M = Nb or Ta. Here the metal-alkene complex is relatively quite stable, so much so that the kinetics of formation of a stable alkyl metal complex cannot be easily studied. Nonetheless it is possible to get a handle on the hydrometallation kinetics by means of dynamic NMR methods (Table 1). The increased rate for propene versus ethylene results from both steric destabilization of the ground state and electronic stabilization of the transition state for the former. The first is typical of alkene complexes the second implies that some partial positive charge develops at the -carbon during the hydrometallation process, also seen in the trend for substituted styrenes. 

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