Variable hapticity

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Many organometallic ligands are capable of variable hapticity. Earlier we saw that the allyl ligand can bond either η1 or η3 to a metal. In cis-allylbromobis(dimethoxyethane)magnesium, sterics dictate that the allyl ligand bonds η1 to the magnesium ion (see OKUSES). In contrast, there is sufficient space in η3-allylchloro(triphenylphosphine)palladium(II) for the allyl ligand to bond η3 to the palladium ion (see ALPHPD01). It is even possible to isolate compounds where the same type of ligand bonds with variable hapticity to the same metal. A fine example is the compound (η3-allyl)bis(η1-allyl)(1,2-bis(diphenylphosphino)benzene)iridium(III) (see WIZKEV).

•	The cyclopentadienyl ligand, C5H5- , or Cp- is also versatile in it’s bonding modes, examples include CSD refcodes CACWOS, PEJGAM, and MULJIM. For each of these structures identify the mode of bonding of the Cp- ligand.