FIVE-ELECTRON LIGANDS

The cyclopentadienyl group, C5H5, forms complexes with all the d-block transition elements. Its usual mode of bonding is fy , although there are examples of and f] attachment. In the following series of compounds the 18-electron configuration of the metal is maintained by variation of the number of electrons provided by one of the cyclopentadienyls (Fig. 9.1). 

While the fy -cyclopentadienyl ligand can undergo reactions such as electrophilic substitution at the ring (p.283), it is often rather inert. It can therefore play the part of a bystander, while changes are going on at the metal centre or at other ligands in the molecule. It effectively occupies three 

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C. Complexes Having Five-Electron Ligands Borabenzene (rf-CsB)... 

B. Substitution of Two-, Three-, Four-, and Five-Electron Ligands... 

A number of approximate molecular orbital calculations on ferrocene were reviewed by Cotton and Wilkinson in 1959 and the result of an ab-initio calculation was published in 1972 The latter work also contains references to approximate molecular orbital calculations published between 1959 and 1972. It seems that the particular stability of ferrocene is due to its closed-shell structure all occupied molecular orbitals are bonding between the metal and the ligands, the cyclopentadienyl rings function as five-electron ligands, and the iron atom attains an inert gas configuration of 18 electrons in the valence shell. 

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