Reactions of unbridged zirconocene dichlorides, (R C H ) ZrCl (n=0, 1, or 2), with diisobutylaluminum hydride (HAI'Bu ) result in the formation of tetranuclear trihydride clusters of the type (R C H ) Zr(μ-H) (Al Bu2)3(μ-CI) 2, which contain three [Al Bu ] units. Ring-bridged ansa-zirconocene dichlorides, Me2E(R C H ) ZrCl with E=C or Si, on the other hand, are found to form binuclear dihydride complexes of the type Me2E(R C H ) Zr(CI)(μ-H) Al Bu with only one [AIBu ] 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 -ring ligands. Alkylaluminum-complexed zirconocene dihydrides, previously observed in zirconocene-based precatalyst systems activated by methylalumoxane (MAO) upon addition of HAI'Bu or Al Bu , are proposed to be species of the type Me Si(ind) Zr(Me)(μ-H) Al Bu , stabilized by interaction of their terminal Me group with a Lewis acidic site of MAO. © 2008 American Chemical Society. n 5 5-n 2 2 2 n 5 5-n 2 3 2 n 5 4-n 2 2 n 5 4-n 2 2 2 2 5 2 3 2 2 2 2 i i i i i