The ethylene-bridged pseudo-Cs symmetric ansa-zirconocene complexes,(1-fluorenyl-2- cyclopentadienylethane) zirconium dichloride (5a) and (1-fluorenyl-2- tetramethylcyclopentadienylethane) zirconium dichloride (5b), were synthesized and their molecular structures have been determined by single crystal X-ray diffraction studies. The complex 5a is very active toward propylene polymerization in the presence of ...