Cumulene ligands

Otherwise, treatment of the chromium complex 10 with an excess of [W (C0)5(THF)] afforded the tungsten allenylidene 61 by transmetallation of the cumulenic ligand and further addition of W(CO)s to the A-atom of the heterocyclic substituent (Scheme 18) [30]. The related chromium complexes [Cr... 

Extending the carbon chain of vinylidenes by one or three carbons provides linear cumulenated ligands, LwM=(C=C)w=CR2 (n = 1,2). These compounds have been studied in considerable detail, especially allenyl-idenes (n = 1) and arise in most cases from the rearrangement and dehydration (often spontaneous) of hydroxyalkyl alkynes or diynes (Figure 5.51). Various intermediates (Figure 4.18) may be envisaged (and occasionally isolated) which are related by elimination/addition of H+ or hydroxide, e.g. y-hydroxyalkynyl complexes have been isolated and subsequently dehydroxylated with Lewis or Bronsted acids. 

This article is prompted by developments in the chemistry of allenyl complexes their systematic synthesis, structural diversity, and patterns of chemical reactivity. The increasing attention being focused on allenyl and allenylidene compounds can be attributed to several factors an upsurge in interest in cumulene ligands in general and electronic communi-... 

As we have noted in a previous section of this chapter, cumulenic resonance forms present one extreme description for dimetal complexes linked by all carbon bridges. They are frequently encountered in some oxidation state (mostly the dioxidised one) and also contribute to intermediate ones between the purely cumulenic and oligoynediyl or the cumulenic- and the alkynyl-bridged dicarbyne forms (see Schemes 6.6-6.8). Purely cumulenic wires are encountered in ot,a)-di- or -tetraferrocenyl substituted cumulenes, but no evidence other than a splitting of the ferrocenyl-based redox waves has been presented to support the presence of electronic interactions between the fer-rocenyl end groups across the cumulenic ligands. Based on the results of ex-... 

The longest cumulenylidene ligand reported to date is the heptahexaenylidene complex shown in Figure 13.49. As in the case of extended organic pi systems, the difference in energy between the HOMO and the LUMO decreases as the length of the cumulene ligand increases. ... 

There is now a growing literature of nickel organometallic complexes that contain carbon dioxide or related cumulene ligands that result from reactions with carbon monoxide. The first structurally characterized complex of carbon dioxide was the nickel complex Ni(G02)(PCy3)2 reported in 1975. A more recent study of this complex provides the complete assignments of the vibrational spectra and theoretical calculations of different isomers in support of a mechanism for CO2 fluxionality that involves end-on coordination. The tridentate pincer ligand 2,6-bis((diiso-propylphosphino)methyl)phenyl (PGP) has been used to form the square-planar Ni(ii) hydroxide complex Ni(OH)(PGP). The complex Ni(OH)(PCP) reacts with CO to give a binuclear /X-GO2 complex (Equation (2)). 

As already mentioned, CAAC ligands can stabilize electron-deficient metal centers such as in cationic gold complexes. Complex 29 catalyzes a very unique reaction of enamines with acetylene, which produces a cumulene and an imine... 

In this chapter, the most efficient synthetic routes, the main stmctural features as well as reactivity patterns of odd-chain metallacumulene complexes bearing 7i-donor substituents, i.e., [M]=C(=C) =CR R ( = 1, 3, 5 R /R = NR2, OR, SR, SeR), are reviewed. In addition, the coordination chemistry of phosphonioace-tylides (R3P C=C ) and tricarbon monoxide (C3O) will also be discussed since these heteroatom-containing 77 -carbon ligands lead to closely related bonding situations, with participation of both neutral cumulenic and zwitterionic alkynyl-type mesomeric forms (Fig. 3). 

Aspects related to the chemistry of the heteroatom-terminated -carbon ligands R3P C=C and C3O have also been discussed. Thus, upon coordination, the former seem to present a partial cumulenic character [M]=C=C=PR3, but little is known about the chemical behavior of this coordinated unit. In the case of the tricarbon monoxide ligand, recent theoretical calculations have shown that coordination chemistry could be an alternative to stabilize this highly unstable heterocumulene. However, the access to metal complexes containing the C3O unit represents an exciting experimental challenge for the near future. 

Phenylmethylsilanediol, synthesis, 42 155 Phenylsilanetriol, monosodium salts, 42 169 4 -Phenyl-2,2 6, 2 -terpyridine bis nickel complex, 30 74 molecular structure, 30 74 PhjtfluorenyllSiOH, 42 197 (Ph(Me2N)C-=Nli], 37 59-65 orientation of imino ligand, 37 61-63 (PhMe SiljCSiH OH, 42 244-245, 248 (PhMe SiljCsiMeHlOH), 42 191 Phosphaalkenes acyclic, 33 338-353 butadienes, 33 346-349 cumulenes, 33 352... 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Coupling of the Carbene Ligand with Cumulenes and C—X tr-Bonds 1107... 

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