Cyclopentadienyl, Allyl, and Related Ligands

Cyclopentadienyl (C H = Cp) is a versatile spectator ligand, and the phenomenal growth of organometallic chemistry of d and / elements owes much to it. Some of the complexes with two cyclopentadienyl ligands are often referred to as metallocenes. In recent times metallocenes and half-metallocenes (one cyclopentadienyl ligand) have been widely tested as single-site catalysts for alkene polymerization reactions. 

Stmctures 2.43 and 2.41 are examples of metallocene complexes. As shown by 2.43, metallocenes of the M elements without any other additional ligands have a common structure. Because of the presence of double bonds, the ligand Cp, like benzene, butadiene, COT, and 1,5-COD, acts both as an acceptor and as a donor. Electron density is donated from its filled r-orbitals. Electron density from the metal d orbitals is accepted in its antibonding r-orbitals. 

In each Cp, there are five p orbitals and as shown for structure 2.43, in the most common rf-mode of bonding, they all are used. The energies of the five highest occupied molecular orbitals (HOMOs) of Cp M complexes are very close to each other. Based on the electron occupancy of these orbitals, magnetic properties and variation in the distances of the centers of the Cp rings from the metal atoms can be easily explained. 

Note that for aromaticity according to the Hiickel rule, An+2 number of p electrons in a cyclic planar structure must be present. By assuming Cp to be Cp, this condition is satisfied. Indeed, many of the properties of Cp complexes, e.g., Fridel-Craft acylation of the ligand in Cp Fe, provide excellent evidence in support of such a formulation. 

Structures 2.43-2.46 illustrate a few other fascinating structures that Cp complexes may adopt. As we will see, by using a few simple rules of electron counting (see Section 2.2.1), many of these structures are easily rationalized. 

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