A possible rationalization of the dependence of regiospecificity of propene polymerization for catalytic systems based on C-2-symmetric ansa-zirconocenes (and hafnocenes) on the pi-ligand alkyl substitutions is presented. With this aim, models of preinsertion intermediates and transition states for primary and secondary monomer insertion reactions are compared through molecular mechanics analyses. The nonbonded energy contributions to the regioselectivity, as well as the enantioselectivities of regioregular and regioirregular insertion steps have been investigated. Nonbonded energy interactions are able to account for the increases of regiospecificity experimentally observed for zirconocene-based catalytic systems when the pi-ligands are alkyl substituted in position 2 or 3 as well as the decrease of regiospecificity observed when the pi-ligands are dimethyl substituted in positions 4 and 7.