TRANSITION METAL COMPLEXES OF CYCLOPENTADIENYL FUSED HETROCYCLES.pptx

Cyclopentadienyl

1. TRANSITION METAL COMPLEXES OF CYCLOPENTADIENYL FUSED HETROCYCLES

2. TOPICS... Introduction Structural types Bonding Reactivity Properties Applications

3. Introduction... Cyclopentadienyl group ( C 5 H 5 ), 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: Half sandwiched OR Piano Stool. Metallocene OR Sandwich. Bent metallocene. Bimetallic half sandwich. Multiducker compounds.

5. 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. 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. Bent metallocene : Bent metallocene ( Cp 2 ML y ) has two Cp ring in a non parallel position and are slightly bent with an angle. CpML y

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 (ψ 1 –ψ 5 ) residing in three different energy levels as shown. The lowest energy orbital ψ 1 is represented by an A 1 state which does not contain any node, followed by doubly degenrate E 1 states thar comprise of ψ 2 and ψ 3 orbitals having a single node, and another doubly

8. Degenerate E2 states consisting of ψ 4 and ψ 5 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 dz 2 interact with A 1 type orbital of Cp ligand, while dyz and dxz overlaps with the E 1 symmetry orbitals and the metal orbitals dx 2 -y 2 and dxy overlaps with E 2 type orbitals having similiar symmetry.

10. In the Cp 2 M 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 A 1g , A 1u , E 1g , E 1u , E 2g , E 2u ligand group orbital.

11. 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. Electrophilic Substitution At Ring : This is a very facile process, occuring at some 3*10 6 times faster than with the benzene.

12. This reaction does not work if the electrophile is an oxidizing electrophile such as NO 2 + These reagents does not oxidize the complex to give the ferricinium cation [FeCp 2 ] + and the positive charge now on the ring makes the electrophile attack very difficult. Reactivity of attached organic groups : Organic functional group attached to Cp undergo may of the most common reaction types..

13. Stabilization of α-carbonium ion : The hydrolysis of aceto substituted ferrocene proceeds seven times faster then the solvolysis of Ph 3 C-OAc ( this is the classical example of stable carbonium ion Ph 3 C and the trityl ion ). So we can say that the carbonium ion intermediate is more stable than the trityl cation.

14. 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..

15. Properties of Cp :

16. 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.

17. References : www.chem.gla.ac.uk>documentation http://en.m.wikipedia.org>wiki>cyclopentadienyl complex www.ilpi.com>organometallic>cyclopentadienyl liagands ethesis.nitrkl.ac.in>phd_551

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