1,2-Addition alkylidyne

The synthetic method leading to Nb-alkylidenes and Nb-alkylidynes was particularly successful, due to a quite remarkable difference in the reaction rate of 29 with ketones or aldehydes, vs the subsequent reaction of the alkylidene with ketones and aldehydes (see Scheme 37). The former reaction takes a few minutes at -40°C, while the latter one occurs in hours at room temperature.88 The reaction between 178 and benzaldehyde led to triphenylethylene and the niobyl derivative 184. Due to the difference in reaction rates between a and b in Scheme 37, it was found that the sequential addition of two different ketones or aldehydes to a THF solution of 29 produced a nonsymmetric olefin in a stepwise McMurry-type reaction.84 This is exemplified in the coupling shown in reaction c (Scheme 37). The proposed reaction pathway does not involve the intermediacy of a pinacolato ligand and therefore differs from the mechanism of the McMurry reaction and related reductive couplings at activated metal sites.89... 

Thus (2 ) is prepared by adding [Rh(C2Hlt)2(acac)] to (6), the latter being preformed by treating the mononuclear tungsten alkylidyne compound with [Rh(C0)2(acacl] (6). Similarly, step-wise addition of [Rh(C0)2(n-C9H7)] to r(n-C5H5)(0C)2W = CR] yields ( )(9). 

Despite early reports on the addition of hydroxyl groups to tungsten-alkylidyne bonds [5-7], recent work has showed that the triple-bond remains untouched, at least in the cases involving the reaction of [Mo(=CEt)(N BuPh)3] [13], [W(=CCMe3) (CH2Me3)3] [26, 27] and [Mo(=CCMe3)(CH2Me3)3] with silica [28]. 

Molybdenum(VI) compounds containing Mo—C bonds are a recent addition to the literature. These compounds include both oxo and non-oxo species. The latter have an alkylidyne (alternatively formulated as an alkylcarbido) group which is a bonding equivalent to the oxo group. 

Several questions remain. In particular, why is it that only one alkylidyne ligand appears to be susceptible to attack (addition/insertion). Perhaps the answer to this question lies in a combination of steric and electronic factors associated with the initial insertion/addition products. In some cases these lack any formal W—W bond, the tungsten atoms being in a formal +6 oxidation state, whereas in others the nature of the W—W bond and its accessibility toward entering 7t-acid ligands is at present not well understood. Further studies are clearly warranted. 

Alkylidene and alkylidyne ligands require metal-carbon p bonding in addition to s. This is different, however, form... 

Just as alkylidenes and alkylidynes may bridge M-M bonds, vinyli-denes may also be employed to support metal-metal bonds. This approach has included the addition of preformed terminal vinylidene complexes to unsaturated metal complexes or, occasionally, the direct assembly from terminal alkynes with unsaturated metal complex precursors (Figure 5.50). 

Emax = 4700 Af cm ). Irradiation into these bands led to the observed protonation reactions as expected, since the excited states so produced would have increased electron density on the alkylidyne ligands making them more susceptible to proton addition [Eq. (156)]. The authors further suggested that the MLCT excited states of these complexes possess bent ... 

In one reaction an additional product (276, Cp = C5H5), with a bridging ligand resulting from the coupling of a CO, the alkylidyne, and S, is also formed (183). 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

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