Reactions of unbridged zirconocene dichlorides, (R nC 5H 5-n) 2ZrCl 2 (n=0, 1, or 2), with diisobutylaluminum hydride (HAI'Bu 2) result in the formation of tetranuclear trihydride clusters of the type (R nC 5H 5-n) 2Zr(μ-H) 3(Al iBu2)3(μ-CI) 2, which contain three [Al iBu 2] units. Ring-bridged ansa-zirconocene dichlorides, Me2E(R nC 5H 4-n) 2ZrCl 2 with E=C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me2E(R nC 5H 4-n) 2Zr(CI)(μ-H) 2Al iBu 2 with only one [AIBu 2] unit. The dichotomy between unbridged and bridged zirconocene derivatives with regard to tetranuclear versus binuclear product formation is proposed to be connected to different degrees of rotational freedom of their C 5-ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of HAI'Bu 2 or Al iBu 3, are proposed to be species of the type Me 2Si(ind) 2Zr(Me)(μ-H) 2Al iBu 2, stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO. © 2008 American Chemical Society.