Polyene ligands

Ziegler-Natta catalyst for polymerization of alkenes. Considerable attention has been directed to double-bonded Fischer carbenes of Cr and W, the Schrock carbenes of Ta and Ti, and cyclic polyene ligands of Fe, Co, Cr, and U. Carbonyls of transition metals from groups 6 to 10 of the periodic table include both the monomeric compounds such as Cr(CO)g, Fe(CO)5, Ni(CO)4 and those with two metal groups such as Mn2(CO)io and Co2(CO)s, which is used industrially for hydroformylation. Although their source has not been identified, it has been shown that volatile compounds from landfills contain carbonyls of Mo and W (Feldmann and Cullen 1997). 

Lamberth, C. Murphy, D. M. Mingos, D. M. P. Second Harmonic Generation Properties of Some Coordination Compounds Based on Pentadionato and Polyene Ligands. In Organic Materials for Nonlinear Optics II Harm, R. A., Bloor, D., Eds. Royal Society of Chemistry London, 1991 pp 183-189. 

Metal migration about a tr-complexed polyene ligand... 

Zerovalent transition metal carbonyl moieties may act as electron acceptors, and thus activate coordinated polyene ligands toward nucleophilic attack. Reaction of (C.411<5 )-Fe(CO)3 with KBHEt3 (—80 °C) proceeds via attack at a coordinated carbon monoxide to generate the anionic iron-formyl species 185 (Scheme 47)184. Upon warming to... 

By the end of this chapter you should have an appreciation of the organometallic chemistry of carbocyclic polyene ligands in both spectator and participatory roles. This will include ... 

TT-Bonded ligands can be closed shell, 2n electron donors (n = 1 — 3) such as alkenes, allenes, alkynes, arenes, and polyenes these have been termed even polyene ligands. They may also be open shell, odd polyene ligands such as allyl, pentadienyl, cyclopropenyl, and cyclopentadienyl, which are formally viewed as anionic hgands, donating 2n electrons (n = 1 - 3). 

It is known that (R3P)2NiBr2 readily releases a triphenylphosphine group in benzene 183). The next step in (41) is analogous to that of substitution of polyene ligands in the presence of strong electron donors. A released double bond coordinates the nickel atom to form an intermediate complex (X) so that an intramolecular transfer of the ligand is possible. 

Cyclohexadienyl complexes react with nucleophiles to give 1,3-diene complexes. An example is shown in Eq. 8.17 the arrow refers to the point of attachment of the nucleophile to the polyene ligand. The synthesis of the starting complex by an electrophilic abstraction is also shown this activates the ligand for nucleophilic attack. Once again, directing effects can be used to advantage a 2-OMe substituent directs attack to the C-5 position of the cyclohexadienyl, for example. ... 

Multipalladium assemblies sandwiched by unsaturated hydrocarbon ligands have been developed by Kurosawa and Murahashi. A general synthetic scheme involves the reaction of 2 equivalents of Pd and (ra 2) equivalents of Pd , with two linear polyenes having (n+1) C = C bonds to construct a linear Pd skeleton (Scheme 39). A homoleptic solvated Pd dimer Pd2(CH3CN)6f has been employed as a monovalent palladium precursor. The polyene ligand coordinate to the Pd chain in r fashion. For polyene with n... 

In solution, facile slippage of the polyene ligand along the Pd4 chain is observed based on NMR study that exhibits averaged 4 vinyl proton signals even down to —90°C (Scheme 41). If the motion is not rapid enough on the... 

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