om 4 to about 20 carbon atoms, R3 is a bridging group, ???n??? is the number of atoms in the direct chain between the two ligands, and is 0 to 8, preferably 0 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 table of the elements and is preferably in its highest oxidation state, Y is a heteroatom containing group in which the heteroatc