The ansa-indene compound {1-Me2Si(3-C9H6Et)(2)} (1) was prepared by alkylation of the unsubstituted ansa-indene. This compound was converted. by reaction with nBuLi, to the dilithium compound [Li-2{1-Me2Si(3-C9H5Et)(2)}] (2). ansa-Zirconocene [Zr{1-Me2Si(3-eta(5)-C9H5Et)(2)}Cl-2] (3) was prepared by the reaction of ZrCl4 with 2 in ether/toluene at -78 degrees C. The molecular structure of meso-3 was determined by single crystal X-ray diffraction studies. The ansa-zirconocene 3 exhibits a greater activity in ethylene polymerization than reference complexes such as [Zr{1-Me2Si(eta(5)-C9H6)(2)}Cl-2] and [Zr{1-C2H4(eta(5)-C9H5)(2)}Cl-2] and, in addition, it maintained a reasonable level of activity after 12h of contact with MAO solution. Furthermore, the different elementary steps in the activation process of ethylene polymerization for substituted complexes [Zr{1-Me2Si(3-eta(5)-C9H5R)(2)}Cl-2] (R=Et 3, Me 4, nPr 5 and nBu 6) by commercial methylaluminoxane (MAO) have been studied by UV-vis spectroscopy. Addition of MAO in large excess ([Al]/[Zr] = 2000) at -78 degrees C yields a previously unreported intermediate in the activation process of metallocenes: this intermediate has an absorption band centered at lambda = 639 nm. We report here the influence of the type of catalyst, ring substitution, type of cocatalyst and addition of THF on the activation process of these metallocenes. (c) 2006 Elsevier B.V. All rights reserved.