Catalysts useful for polymerizing olefins are disclosed. The catalysts comprise an activator and a triple-decker bimetallic complex. The complex includes two Group 3-10 transition metals and a delocalized dianionic ligand that is pi-bonded to each of the metals. The behavior of the catalysts can be modified by choice of each metal, by the choice of the dianionic ligand, or by choice of the ancillary ligands. The invention provides a new way to make a large variety of catalyst systems.