The formation of the precursors of the polymerization-active species of the metallocene dichlorides Cp2ZrCl2 and Ph2C(CpFlu)ZrCl2 by successive reaction with AlBu31 and [PhNMe2H][B(C6F5)4] was investigated by means of NMR spectroscopy. More than two equivalents of AlBu31 are required for total conversion of the metallocene dichlorides in the first step. The reaction of Ph,C(CpFlu)Zr-Cl-2 with AlBu31 leads exclusively to the mono-iso-butyl complex Ph2C(CpFlu)ZrClBu1, independent of the surplus AlBu31 used, whereas in the case of Cp2ZrCl, a series of metallocene products are observed, depending on the Al/Zr ratio used. When this ratio was increased to above 10. the reaction could be exclusively directed to form the dimer metallocene complex [Cp2ZrH2.AlBu31](2). The reaction of [PhNMe2H][B(C6F5)(4)] with metal- locene/aluminium alkyl mixtures prepared with 10, 20, 50 and 100 equivalents of AlBu31 leads to the precipitation of an oily liquid, which contains resulting cationic metallocene complexes. These liquid phases can be purified by extraction and subsequently used for NMR measurements. With one exception, mixtures of two or three different cationic metallocene products are obtained, depending on the Al/Zr ratio and on the metallocene ligand used. An Al/Zr ratio of 100 the reaction of the Ph2C(CpFlu)ZrCl2/AlBu31 mixture with [PhNMe2H][B(C6F5)(4)] exclusively leads to the cationic heterodinuclear metallocene complex [Ph2C(CpFlu)Zr-mu-H-mu-(C4H7)-AlBu21](+), a novel type of allyl-bridged cation, which was characterized by NMR data. None of the reactions of metallocene dichloride/aluminium alkyl mixtures with [PhNMe2H][B(C(,F)4] lead to the degradation of [B(C6F5)(4)](-), whereas in the absence of metallocenes AlBu31 reacts with [PhNMe2H][B(C6F5)(4)] to give AlBu3-x1(C6F5)(x) compounds. Based on these results and with additional information from the literature a mechanism is proposed to explain the formation of [Ph2C(CpFlu)Zr-mu-H-mu-(C4H7)-AlBu21](+). (C) 2002 Published by Elsevier Science B.V.