_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

9
Five electron ligands
9.1 lntroduction
The cyclopentadienyl group, C 5 H 5 , forms complexes with all the d-block
transition elements. Its usual mode of bonding is IJ 5 , although there are examples
of IJ 3 and IJ 1 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 1] 5 -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
Fig. 9.1 Various bonding modes in cyclopentadienyl complexes.
~ ~ ~ ~
,M ,M ,M""-. ,M""-.
c/i ""-c c"J ""-c c"J c c"'i c
o c o o c o o c o o c o
o o o o
Fig. 9.2 11'-Cyclohexadienyl, 17'-cycloheptadienyl, 1]'-pentadienyl analogues of 1] 5 -
cyclopentadienyltricarbonylmanganese.
278