Bridging ligands Alkylidene

Finally, bridging ligands may be classified using a similar approach. For example, a bridging alkylidene is classified with the notation [X-/x-X], while a bridging chloride or acetate ligand would be represented by [X-p-L] (Figure 26). " ... 

B-ddazoketones also yield a-alkylidenes or bridging ligands incorpor-... 

Fiirstner and coworkers [181] report heterobimetaUic gold(l)-chromium(0) and gold(l)-tungsten(O) complexes bridged by alkylidene ligands. Scheme 11.9. Complexes of both group 6... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

Treatment of alkylidene-bridged zirconium—aluminum species with HMPA activates the C—A1 bond of the alkylidene unit, making it susceptible to electrophilic attack [146]. Ligand-based activation of the C—A1 bond can also be used to convert alkylidene-bridged zirconium-aluminum reagents to other bimetallic species. Thus, treatment of 3 with HMPA followed by addition of a weakly electrophilic metal salt can give rise to a new heterome-tallic species. Slow addition of a solution of R3SnCl in toluene to a solution of 3 and 1... 

A notable double insertion of methylacetylene into the methylene bridge has been observed with the classical -y-lactone-type cobalt complexes 1. The heterocyclic methylene ligand (CRR ) originally attached to both metal centers is displaced by the new alkylidene ligand ju.-C(CH3)— C(H)=C(CH3)—C(H)=CRR (116, 122) (Scheme 28a other ligands omitted for clarity). This example also points to the possibility of methylene homologation in the coordination core of a catalyst (see Section IX). 

R CN (Table 2) [86]. The molecular structure of the 2,2-dimethylpropanonitrile derivative contains unsymmetrically bridging alkylidene amide ligands. Reaction of the yttrium and erbium hydride species with isonitrile results in the formation of a formidoyl moiety (Table 2) [87], Surprisingly the Ln-N interaction is in the range of the nitrile product A similar molecular structure was found in the oximato complex [Cp2Gd(/i-t/2-ONCMe2)]2 (Gd-Nav 2.42(1) A) [88]. 

The bridging alkylidene ligand in 12 has been found to be a reactive species towards several small molecules. The addition of dihydrogen (45 psi, 50°) to 12a resulted in cleavage of the alkyl -idene from the metals diphenylmethane (confirmed by XH NMR spectroscopy and GC/MS) and 1 were obtained in high yield (eq. 34). 

COMPOUNDS WITH BRIDGING ALKYLIDENE AND ALKYLIDYNE LIGANDS. 

The results described in this paper suggest that further studies in this area would be very profitable. The heteronuclear metal —metal bonded species will have a rich chemistry at the metal centres, particularly in those cases where bridging alkylidene or alkylidyne ligands are present. 

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