Ethylene polymerization using catalysts derived from activation of zirconocene aluminohydride complexes with either methyl aluminoxane or B(C6F5)3 is reported. Variable-temperature NMR spectra of mixtures of Cp*2ZrH3AlH2 or Cp‘2ZrH3AlH2 and excess B(C6F5)3 reveal the formation of di- or polynuclear metallocenium ion-pairs featuring terminal or both terminal and bridging borohydride counteranions HB(C6F5)3 arising from hydride abstraction. At higher T, ion-pairs featuring the terminal HB(C6F5)3 counterion decompose, and the AlH3 that is liberated degrades B(C6F5)3 to furnish mixtures of (C6F5) n AlH3 - n and, in the case of Cp*2ZrH3AlH2, a new ion-pair partnered with the diborohydride counteranion [Cp*2ZrH][(μ-H)2B(C6F5)2]. The latter compound was independently prepared from Cp*2ZrH2 and HB(C6F5)2 and is active in ethylene polymerization; however it is 1000 times less active than the catalyst formed from Cp*2ZrH3AlH2 and B(C6F5)3 and so cannot account for the multisite behavior of the latter combination. There is evidence of chemical exchange between “free” or terminal HB(C6F5)3 and excess B(C6F5)3 in these mixtures, and on the basis of model studies with [ n Bu4N][HB(C6F5)3] and B(C6F5)3, this involves reversible formation of [ n Bu4N][(C6F5)3B)(μ-H)B(C6F5)3], which can be detected by 19F NMR spectroscopy in solution at low T.

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