Cp2M + species, formation

Harrod s preliminary suggestions for the dehydrocoupling of hydrosilanes by early transition metal catalysts involved the formation of metal-silylene intermediates, Cp2M=SiRR [138a]. The most plausible mechanism for the condensation reaction, however, has been presented elegantly by Tilley and co-work-ers [144] and involves ff-bond metathesis from M-H species generated from the catalyst precursor. 

The true active species in metallocene-MAO systems are believed to be metallocene alkyl cations, i.e., cationic electron complexes of the type [Cp2M(R)] (M = Ti, Zr, Hf). As shown below, the formation of the cat-alytically active complex involves a series of reactions between metallocenes and MAO [43 j. For the halogen-containing metallocenes, a rapid alkylation of metallocene by MAO takes place first, and the active species arises from a methyl transfer reaction between the metallocene alkyls and MAO. 

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