3. Introduction...
Cyclopentadienyl group (C5H5), the deprotonated form of
cyclopentadiene, has been one of the versatile and widely used ligand
in organometallic chemistry. It is a monoanionic, aromatic, electron
rich species containing six-electrons and form a variety of complexes
with the main groups and transition metals. The cyclopentadienyl
group is a significant ligand because of its aromaticity and high
stability and it plays an important role in stabilizing organometallic
complexes.
In electron counting it can be treated as 6-electron donor anion or 5-
elctron donor neutral molecule.
4. Structural types Cp compounds...
Structural varieties of Cp based complexes are as follows:
1. Half sandwiched OR Piano Stool.
2. Metallocene OR Sandwich.
3. Bent metallocene.
4. Bimetallic half sandwich.
5. Multiducker compounds.
5. 1. Half sandwich :Half sandwich complexes ( ) sometimes
also referred as “Piano stool” complexes. These complexes have one
cyclopentadienyl moiety bonded to the metal unit.Depebding upon
the number of ligands L bound to the metal center, the complexes
can be named as two leg, three leg, etc... Piano stool complexes.
1. Metallocene : In this the metal remains in between the two
cyclopentadienyl rings. The two cyclopentadienyl rings are known to
form staggered or eclipsed conformations with a range of different
torsional angles.
1. Bent metallocene : Bent metallocene (Cp2MLy) has two Cp
ring in a non parallel position and are slightly bent with an angle.
CpMLy
6. 4. Bimetallic compounds : These compoundes have more than
one metal-Cp fragment bpound together by one or more than one
metal-metal bonds.
5. Multidecker compounds : These compounds are relatievely
less known than other meal-Cp complexes but requires prime
attention because of their unique structural identity and complicated
synthetic process.
7. Bonding in cyclopentadienyl compounds :
Cyclopentadienyl ligand can adopt pentahapto, trihapto, and
monohapto coordination modes with metals. Among the three
coordinations pentahapto is the most common mode of bonding forming
a six electron donating ligand.
The frontier molecular orbital of the cyclopentadienyl ligand
contains five orbitals (ψ1–ψ5) residing in three different energy levels as
shown.
The lowest energy orbital ψ1 is represented by an A1 state which does
not contain any node, followed by doubly degenrate E1 states thar
comprise of ψ2 and ψ3 orbitals having a single node, and another doubly
8. Degenerate E2 states consisting of ψ4 and ψ5 orbitals having two nodes. The energy of
states increases as the number of nodes increases from 0 to 2.
The atomic orbitals of metals which are involved in the interaction with the orbital of Cp ligand
to form Metal-Cp molecular orbital are shown..
9. The metal d-orbitals which are mainly found to overlap with the Cp-MO’s having the
matching symmetry are shown below..... The orbital dz2 interact with A1 type orbital of
Cp ligand, while dyz and dxz overlaps with the E1 symmetry orbitals and the metal
orbitals dx2-y2 and dxy overlaps with E2 type orbitals having similiar symmetry.
10. In the Cp2M system each of these five orbitals of the two Cp ligands combines to give
ten ligand molecular orbitals in these energy levels. All these ligand group orbital on
combination gives rise to A1g, A1u, E1g, E1u, E2g, E2u ligand group orbital.
11.
12.
13. The resultant MO scheme for FERROCENE shows the way that the basis orbitals
having the same symmetry can combine to give new orbitals.
The filling of the nine bonding orbitals in ferrocene explains the high stability of this
compound.
Reactivity of Cp type ligands :
Cp is a spectator ligand (i.e. It does not take part in the reaction ). But under some
circumstances the ring will react. Ferrocene is the most studied reaction as it is stable
18e metallocene.
1. Electrophilic Substitution At Ring :
This is a very facile process, occuring at some 3*106 times faster than with the
benzene.
14. This reaction does not work if the electrophile is an oxidizing electrophile such as NO2
+
These reagents does not oxidize the complex to give the ferricinium cation [FeCp2]+ and
the positive charge now on the ring makes the electrophile attack very difficult.
2. Reactivity of attached organic groups :
Organic functional group attached to Cp undergo may of the most common
reaction types..
15. 3. Stabilization of α-carbonium ion :
The hydrolysis of aceto substituted ferrocene proceeds seven times faster then the
solvolysis of Ph3C-OAc (this is the classical example of stable carbonium ion Ph3C
and the trityl ion ). So we can say that the carbonium ion intermediate is more
stable than the trityl cation.
16. 4. Heterocycles as Cp analogues :
The CH group in Cp can be replaced by hetero atoms to gives heterocycles..
These anions form л- complexes which are similiar to Cp..
18. Applications of Cp :
➢ Cp metals complexes are mainly used as stoichiometric reagents in chemical
research.
➢ Ferrocinium reagents are used oxidents.
➢ Cobaltocene is a strong soluble reductant.
