Bonding to Olefins, Polyolefins and Alkynes

1. Bonding to Olefins, Polyolefins and Alkynes Phan Nguyen HuuTrong 201351014

2. Contents: • Olefineand polyolefin complexes • Alkenes • Dienes • Allyls • Cyclobutadienes • Cyclopentadienes • Arenes • Alkyne complexes

3. Olefins & polyolefins complexes

4. Zeise’s salt Zeise’s reaction • Both, Cl- ion and ethylene are coordinated with Platinum ion, hence inside the coordinations sphere

5. Zeise’s salt • The PtCl3 moiety forms a nearly planar group with Pt atom. • Pt-Cl bond trans to ethylene group (2.34 Ao) is significantly longer than cis Pt-Cl bonds (2.302 and 2.303 Ao). • C atoms are approximately equidistant from the Pt atom (2.128 and 2.135 Ao). • Back-bonding is also indicated by a bending of the four hydrogen atoms away from the Pt atom.

6. Dewar-Chatt model • Donation of C=C π electrons to an empty d orbital on the metal. • Back donation from a metal d π orbital into the empty π* orbital of the ligand.

7. Alkene complexes • Factors favoring X2-type binding • Strong donor ligands • A net negative charge on the complex • Low-oxidation state metals • Reactivity differnece: • L-type: alkene is electrodeficient and prone to attack by nucleophiles • X2-type: the carbons are carbanion-like and prone to attack by electrophiles.

8. Alkene complexes • Both cases considered as a 2e donor in the covalent model. • In the ionic model, X2 has a 2- charge and considered a 4e donor. • One can consider L as an intermediate structure in the oxidative addition of alkeneligand to form X2 • The C atoms of the alkenerehybridize close to sp3

9. Dienes complexes • 4e donor • The substituents at C1 and C4 twist approximately 20o-30o out of the plane • C1 and C4 are closer to the metal

10. Dienes complexes Fe(4-C4H6)(CO)3 -donation from alkenes to the Fe weaken and lengthen the C=C bond Zr complex, the single bond is quite shorter a bit. Zr is electron-rich that it transfers two electrons to butadiene via π-backdonation and generate metallocyclopentene.

11. Electrons effects • The more π-backbonding the weaker C=C bond and lower the C=C stretching frequency in IR. • Ag+ complex is believed no π-backbonding

12. Metal-olefin bond strenth

13. Allyl complexes

14. Allyl complexes

15. Cyclobutadien • Cyclobutadien is too unstable to exist, highly reactive, antiaromatic and rectangular. • But the ligand is square and aromatic. • Receiving 2e from the metal to form 6πe aromatic system

16. Cyclopentadienyl complexes • Cyclopentadienyl group is important because it is the most firmly bond and most inert to electrophilic and nucleophilic reagent. • This makes a whole series of complexes CpMLn (n = 2, 3, 4) Ferrocenes

17. Cyclopentadienyl complexes

18. Mono-cyclopentadienyl complexes • Exist for every transition metal. • Structure of half-sandwich complexes is often described as “piano stool” geometry.

19. Metallocenes 0.06 Ao longer than in ferrocene. Highly sensitive to air. Oxidation give [CoCp2]+ is much more stable Triplet ground state. Cr-C bond 0.11 Ao longer than in ferrocene 20e- complex 0.13 Ao longer than in ferrocene.

20. Bent sandwich complexes • Bent metallocene complexes are restricted to metals with a low number of d-electrons those of group 4 and of the heavier elements of groups 5-7

21. Arene complexes • Arenes usually bind to transition metals in 6e, 6 form, but 4 and 2 structures are also known. • C-C distances are usually essentially equal, but slightly longer than in the free arene. • Arenes are much more reactive than Cp group, so easily lost from the metal.

22. Arene complexes 6 tends to be flat. 2 4 is strongly folded

23. Bis-Arene complexes • Synthesis from atomic metal vapor by co-condensation : • Ligands are no negatively charged, • Bis-arene complexes are in general less stable than metallocenes and more eaisly oxidized

24. ALKYNES COMPLEXES

25. Alkyne complexes • Alkynes are essentially like alkenes • One more pair of π-electrons. • They can act as neutral 2 or 4e- donors. • Much better bridging ligands.

26. Alkyne complexes • More electronegative, encourage back donation and bind more strongly • The substituents tend to fold back away from metal by 30o-40o in the complex • M-C distances are slightly shorter than alkenes

27. Bridging metal alkyne complexes

28. Tautomerization

29. Benzyne complexes

30. Reactivity of alkynes

31. References • Lecture of part 2 metal complexes of π-ligand , CH328 Organometallic Chemistry, Wilfrid Laurier University. • George G. Stanley, Lecture notes of organic chemistry, chem 4571, Louisiana State University, 2008. • Robert H. Crabtree, the organometallic chemistry of the transition metals, chapter 5: complexes of π-bound ligands, John Wiley & Son, 2005.