Ligands alkylidene

There have been few previous reports of complexes containing the secondary alkylidene ligands —CHMe and =CH-p-tolyl, and 50 is the first characterized complex to contain the carbethoxymethylene ligand, =CH(C02Et). 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The initial site of protonation in these reactions has not been unambiguously determined. Alkyl ligand formation by protonation at the metal followed by a rapid 1,2-proton shift to the alkylidene ligand is equally as plausible as direct protonation at Ctt. As the metal electron density... 

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Few cationic iron carbene complexes have been characterized spectroscopically [55,104,464], because most such compounds quickly degrade at room temperature [466]. Besides elimination of the alkylidene ligand, one major decomposition pathway can be disproportionation [459] (Figure 3.14). 

Recent developments have been reported for imido-molybdenum systems. When X = 2,5-dimethylpyrrolyl, more stable surface species can be obtained (although a second minor species, formed by the addition of a surface hydroxyl group to the alkylidene ligand, is also present). Moreover, by tuning the imido ligand, the catalytic activity has been improved dramatically (Table 11.3, bottom rows) [20]. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

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). 

In decomposition reactions of dimethyl-metal complexes of palladium(II) and nickel (II) one finds the formation of only traces of methane [49] which may also attributed to an a-elimination process. In regard to the valence state, note that, formally, the alkylidene ligand is considered as a neutral ligand and therefore, in the tantalum-alkylidene complex in Fig. 4.29, tantalum is trivalent. The electronic structure of the alkylidene is of course reminiscent of the corresponding oxide CpTa(Cl)20, which we would definitely call pentavalent. All that matters is that there should be a sufficient number of electrons for the multiple bonds which we draw. 

---