Stability of Polyene and Polyenyl Complexes

The stability of the polyene complexes L toward dissociation is in general Jess iJian iJiat of polyenyl complexes h X. because the free polyene is usually 

Although the problem is less severe for ri -Cp and complexes, these 

Jolly and G. Wilke, The Organic Chemistry of Nickel, Academic Press, New York, 1974. 

Among the products formed from PhC CPh and Fe2(CO)9, is 2,3,4,5,-tetraphenylcyclopentadienone. Propose a mechanism for the formation of this product. Do you think the dienone would be likely to form metal complexes Suggest a cific example and how you might try to make it. 

Suggest a synthesis of Cp2Mo(C2H4)Me from CP2M0CI2. What orientation would you expect for the ethylene ligand Given that there is no free rotation of the alkene, how would you show what orientation is adopted  

Although the problem is less severe for -Cp and (i7 -C6H6) complexes, these are notably less stable on the right-hand side of the periodic (able, for example, for Pd and Pt. The jj -butadiene and ij -allyl groups do not seem to bind less strongly until we reach group 11. 

Stability of polyene complexes also increases in lower oxidation ates. In Eq.5. 54, Co -I) back-donates so strongly that it gives the t -anthracene ligands significant enediyl. (LX2) character.  

Among the products formed from PhCsCPh and Fe2(CO)9, is 2,3,4,5,-tetraphenylcyclopentadienone. Propose a mechanism for the formation of 

L X ligands such as Cp tend to bind more strongly than comparable L ligands such as benzene. 

Increased back bonding to iv-bound ligands (e.g., Sections 5.1-5.3) weakens bonds within the ligand and decreases the tendency for nucleophilic attack on the ligand. 

Claiser, S. Pillet, N. Lugan, E. Despagnet-Ayoub, M. Etienne, and C. Lecomte, Dalton Trans., 41, 6598, 2012. 

Suggest a mechanism for the following transformation and say how you would test it. 

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As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

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