The interaction of (eta(5)-C5H5)(2)ZrX(2) (X = Me, Cl) with Al((t)Bu)(3) and alumoxanes [((t)Bu)(2)Al{mu-OAl((t)Bu)(2)}](2) and [((t)Bu)Al(mu(3)-O)](n) (n = 6, 7, 9) has been investigated The Lewis acid-base complexes (eta(5)-C5H5)(2)Zr(X)(mu-X)-Al((t)Bu)(3) [X = Me (1), Cl (2)] have been isolated and characterized by variable temperature NMR spectroscopy. The molecular structure of compound 2 has been obtained by X-ray crystallography, indicating the presence of a Zr(mu-Cl)Al moiety. The Zr(mu-Cl)Al interaction in compound 2 is compared to the Al-Cl bond in [PPN][AlCl-((t)Bu)(3)] (4). [((t)Bu)(2)Al{mu-OAl((t)Bu)(2)}](2), which contains two three-coordinate (unsaturated) aluminum centers, shows no reaction with (eta(5)-C5H5)(2)ZrMe(2) and no catalytic activity toward ethylene polymerization. In contrast, the closed cage compound [((t)Bu)Al(mu(3)-O)](6) reacts reversibly to give the ion pair complex [(eta(5)-C5H5)(2)ZrMe][((t)Bu)(6)Al-6(O)(6)Me] (7). The temperature dependence of the equilibrium constant K-eq has been determined and, hence, the enthalpy and entropy for the formation of complex 7 [Delta H = -50(1) kJ mol(-1), Delta S = -156(5) J mol(-1) K-1]. Complex 7 is active as a catalyst for the polymerization of ethylene. Polymerization is also observed for mixtures of (eta(5)-C5H5)(2)ZrMe(2) with [((t)Bu)Al(mu(3)-O)](n), (n = 7, 9) despite the lack of observable complex formation. A solution structure of 7 is proposed upon the basis of NMR spectroscopy and a comparison with [(Et(2)O)Li](2)[((t)Bu)(6)Al-6(O)(6)Me(2)] (8), formed from the reaction of [((t)Bu)Al(mu(3)-O)](6) with MeLi in Et(2)O. Upon the basis of NMR spectroscopy, compound 8 exists as either the anti (8a) or syn (8b) isomer as a result of endo or exo methylation of the aluminum centers. The lithium atoms in compound 8 are formally two-coordinate; however, close tert-butyl C-H...Li contacts suggest the presence of agostic stabilization. These results are discussed with respect to the commercial (eta(5)-C5H5)(2)ZrMe(2)-methylalumoxane (MAO) polyolefin catalyst system, and the new concept of "latent Lewis acidity" (Gamma(Al-O)) is proposed to account for the reactivity of the cage hexamer [((t)Bu)Al(mu(3)-O)](6). Crystal data for 2 : orthorhombic, Pnma, a 32.181(9) Angstrom, b = 14.437(4) Angstrom, c = 10.812(3) Angstrom, Z = 4, R = 0.1091, R(w) = 0.1165. Crystal data for 4 : monoclinic, P2(1)/n, a = 15.946(2) Angstrom, b = 18.487(2) Angstrom, c = 16.453(2) Angstrom, beta = 110.778(7)degrees, Z = 4, R = 0.0496, R(w) = 0.0512. Crystal data for 8 : orthorhombic, Pbca, a = 18.249(8) Angstrom, b = 15.215(6) Angstrom, c = 18.359(9) Angstrom, Z = 4, R = 0.0891, R(w) = 0.1190.