New metallocene ligand systems and catalysts/catalyst precursors are described where the ligand system contains at least one open-pentadienyl ligand characterized by an acyclic six .pi. electron delocalized, five atomic center structure. The open pentadienyl ligand is bonded to either another pentadienyl ligand or a Cp ligand by a bridging group. The metal is from group 3-5 or the Lanthanides. These metallocene catalysts/catalyst precursors optionally in combination with co-catalysts can be used to generate a wide variety of different polymer types including HDPE, LLDPE, LHDPE, ethylene/propylene elastomers, tactiospecific C3+ .alpha.-olefins, intimate mixture thereof and the like.