The ethene polymerization activities of a series of Cp'(C5H5)ZrC12 and CpiZrC12 pre-catalysts (Cp' = C5HMe4, C,Me4P, C5Me5, C5H4tBu, C5H3-1,3-tBu2, C5H2-1,2,4-tBu,) together with (C5H&ZrC12 have been correlated with the coordination gap aperture. In the case of the mixed substituted C,R,-,R; ligands, the coordination gap aperture has been determined with the help of the cyclopentadienyl cone angle and the bending angle at zirconium. A hindered rotatability of the fert-butyl substituted systems has been evaluated by molecular modeling calculations and was included in the gap aperture estimation. Deviations from the activity--gap aperture correlation in the case of the phospholyl (C4Me4P) systems could be accounted for in terms of adduct formation between aluminum species and the phosphorus donors. These Lewis acid/base adducts form in an equilibrium reaction at high Al/Zr ratios, as shown by 31P NMR. Ab initio calculations on model LzTiMe+ complexes (L = C5H5, C5H4N and C5H4P) for the insertion of ethene in the Ti-Me bond suggest a high electronic similarity for phospholyl and cyclopentadienyl. The glZr-NMR data (chemical shift and line width) for the above zirconocene series are reported. The glZr chemical shift values increase with a good linear correlation to the number of methyl or tert-butyl groups, which is traced to the additive electron donating effect of the alkyl groups. The hyperconjugative donor effect of a tert-butyl group is found here to be 1.25 times that of a methyl group.The X-ray structure of (C5H3-l,3-tBu2)(C5H5)ZrC12 has been determined (monoclinic, P2,/n, (I = 6.631(3), b = 18.843(9), c = 15.265(5) A, p = 91.3", Z = 4).
