The ansa-zirconocene complex rac-Me2Si(1-indenyl)(2)-ZrCl2 ((SBI)ZrCl2) reacts with diisobutylaluminum hydride and trityl tetrakis(perfluorophenyl)borate in hydrocarbon solutions to give the cation [(SBI)Zr(mu-H)(3)((AlBu2)-Bu-i)(2)](+), the identity of which is derived from NMR data and supported by a crystallographic structure determination. Analogous reactions proceed with many other zirconocene dichloride complexes. [(SBI)Zr(mu-H)(3)((AlBu2)-Bu-i)(2)](+) reacts reversibly with (ClAlBu2)-Bu-i to give the dichloro-bridged cation [(SBI)Zr(mu-Cl)(2)(AlBu2)-Bu-i](+). Reaction with AIMe(3) first leads to mixed-alkyl species [(SBI)Zr(mu-H)(3)(AIMe(x)(i)Bu(2-x))(2)](+) by exchange of alkyl groups between aluminum centers. At higher AlMe3/Zr ratios, [(SBI)Zr(mu-Me)(2)AlMe2](+), a constituent of methylalumoxane-activated catalyst systems, is formed in an equilibrium, in which the hydride cation [(SBI)Zr(mu-H)(3)(AIR(2))(2)](+) strongly predominates at comparable HAI(i)Bu(2) and AlMe3 concentrations, thus implicating the presence of this hydride cation in olefin polymerization catalyst systems.