n adjacent carbon atoms of the cyclopentadienyl ring associated there which can be joined together to form a ring containing from 4 to 20 carbon atoms, R3 is a bridging group, n is the number of atoms in the direct chain between the two ligands and is preferably 1 to 8, most preferably 1 to 3, M is a transition metal having a valence of from 3 to 6, preferably from group 4, 5, or 6 of the periodic