The specifics for binding of π cyclic systems are much more complex and depend on the electrons, the HOMO, and the LUMO in each individual case of molecules. Cyclic π systems can bind monohapto or polyhapto depending on the individual situation. This means that π bonds can bind individually to the metal or there can be a single bond from the center of a benzene or cyclopentadienyl complex. Of course the bonding modes (η, η, η, etc.) determine the number of donated electrons (1, 3, 5, etc.). The diversity of these cyclic complexes allows for a seemingly endless number of metallic structures.