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One of the last hitherto unknown simple metallocenes, the aluminocenium cation Al(η-C5H5)2+ 1, is readily accessible from AlCp2Me and B(C6F5)3. The spectroscopic data are consistent with a metallocene structure. The complex is a highly efficient initiator for the carbocationic polymerization of isobutene and the industrially important copolymerization of isobutene and isoprene.
Bochmann et al. (Fri,) studied this question.