Olefin coordination polymerization catalysts consisting of Group 4 metallocene dichloride have received considerable industrial interest, as they constitute a highly active and selective class of catalysts for [alpha]-olefin polymerization. For zirconocene dichlorides of general formula [Zr{{466} 5 -C 5 H 5-n R n } 2 Cl 2 ], the Zr atom is j 5 -coordinated to cyclopentadienyl rings that may be substituted or not (R = H, alkyl, aryl...).  When reacted with an excess of co-catalyst, the zirconocenium complexes [Zr{{466} 5 -C 5 H 5-n R n } 2 Me] + responsible for the polymerization activity are formed. The catalytic performance of these catalysts is greatly influenced by electronic and steric factors due to the R substituents on the cyclopentadienyl rings.  The aim of this work is to understand how the introduction of various length alkyl chains modified with Si and F atoms alters the coordination environment and the electronic properties of the active metal center, and how these changes may affect the olefin polymerization process. A combination of theoretical methods, such as density-functional theory with natural bond orbital analysis and the quantum theory of atoms in molecules, is used to explore these issues. The results of calculations for zirconocenium complexes with various substituents are reported, and the structure, energy, electron density distribution for the different species are examined, focusing on the factors that might control catalytic activity in order to identify optimal substituents. The nature of possible intramolecular interactions between substituents and the metal, as well as the influence of such interactions on catalytic performance, are also investigated.