Fischer carbyne

By contrast, the Fischer carbyne tnms-bromotetracarbonyl(phcnylmcthyli-dene)tungsten, Br(CO)4W = CPh [140-142], will polymerise monosubstituted acetylenes by a metathesis mechanism [143] but will not give transalkylidynation products. The Fischer carbyne and related carbynes are effective catalysts for the metathesis polymerisation of not only monosubstituted acetylenes, whose polymerisation can be brought about by a number of catalysts [108, 109, 118, 120-125, 128-134], but also of acetylenes polymerised previously by only a few or no catalysts, i.e. even disubstituted acetylenes [112-115, 126, 127, 144-148] and unsubstituted acetylene [19, 20, 42, 119]. 

It was proved that metal carbynes are sources of metal carbenes [e.g. scheme (9) in Chapter 6] promoting the polymerisation of acetylenic monomers. Therefore, related metal carbynes and carbenes appeared to catalyse the polymerisation of alkynes in the same way as regards the identity of the products, in particular as regards stereochemistry. For the terminal and internal alkynes, the Fischer carbyne acts much like the Casey and Fischer metal carbenes. The Fischer carbyne also promotes acetylene polymerisation, and it does this where the Fischer carbene fails and the Casey carbene is much less effective [22,143]. 

Fischer carbyne complexes are usually generated from carbene complexes by 1,2-elimination of an alcohol under the influence of boron trihalides204 or other Lewis acids.205... 

Carbyne ligands can also be grouped into two classes Fischer and Schrock carbynes. The metal-carbyne carbon triple bond (Fig. 3) for a Fischer carbyne complex is formed... 

There are several parallels between the chemistries of the carbene and carbyne ligands. The classification of carbyne complexes into two major structural types—Fischer and Schrock—is perhaps the most obvious of these parallels. Complex 64 represents the prototypical Fischer carbyne complex Ccarbyne bonded to a low-valent metal with Ji-accepting CO ligands attached. Structure 65, on the other hand, is a classic example of a Schrock carbyne complex because a high-valent metal is present with electron-donating ligands attached. Atoms attached to Ccaibyne helpful in distinguishing between Fischer and Schrock carbene complexes (i.e., heteroatoms for the former and H and C for the latter), are less important in the case of carbyne complexes. It is convenient to classify carbyne complexes... 

Molecular Orbital Bonding Scheme for a Fischer Carbyne Complex... 

Equation 10.52 shows the chemistry used to produce 64 and similar complexes this not only is the original procedure employed by Fischer, but also constitutes perhaps the most versatile method for the preparation of Fischer carbyne complexes. The reaction begins with an electrophilic abstraction of the alkoxy group from starting carbene 69 to give the cationic carbyne 70. Carbyne ligands are such powerful n acceptors that they exert a strong trans effect (Section 7-1-1). If... 

As with carbene complexes, metal carbynes display a range of reactivity with electrophiles and nucleophiles. Molecular orbital calculations show that even cationic Fischer carbyne complexes are polarized as M, +=C A neutral Fischer- and Schrock carbyne complexes have an even greater negative charge on Ccarbyne.93 If all reactions between carbyne complexes and other species were charge-controlled, we would predict that nucleophiles would always attack at the metal and electrophiles at Ccarbyne. As we should expect by now, the picture is more complicated in practice. 

LUMOs of a Neutral Fischer Carbyne Complex Showing Only the M-CQlbyne Interactions... 

On the other hand, neutral Os alkylidyne 82 undergoes reaction with methanol to give carbene complex 83 (equation 10.56).96 It would appear that 82 undergoes reaction with nucleophilic methanol at 0 first, which is followed by proton transfer to Os. Such reactivity would be consistent with that associated with Fischer carbyne complexes, yet the metal center is more electron-rich than the group 6 metal complex reactant in equation 10.55. 

Figure 10-12a HUMO of a Cationic Fischer Carbyne Complex 447...

Wadepohl etal.103 also investigated the hydroboration reactions of Fischer carbyne metal complexes [W(=CR)(Tp )(CO)2]. The boryl-metal complexes [W(Tp )(CO)2 ri2-B(R/)CH2R ] (R = Et or Ph, R = CgH4Me-4 or Me) has been prepared. A 3-agostic C-H- -M interaction is present between the metal and the boryl ligand.103... 

The boryl system Tp (CO)2Mo /72-B(Et)CH2(C6H4Me-4), (6.13, Fig. 7), has been synthesized by the reaction of the hydroborating agent Et2BH with the Fischer carbyne complex Tp (CO)2Mo=C(C6H4Me-4) [48,49] a number of related tungsten complexes have been synthesized which are discussed below (vide infra). 

Low-valent (Fischer) carbynes [F(CO)4W=CH],... high-valent (Schrock) carbenes [X3W=CH],... ... 

If L is CO, then the halide ion (Cl, Br, or I) displaces the CO trans to the carbyne in the intermediate cationic complex this shows the high trans effect of the carbyne. On the other hand, if L is PMc3, then the cationic species is the final product. Oxidation states for Fischer carbynes are normally assigned by considering CR as an LX ligand, CR that is, X(CO)4M=CMe is M(II). 

Oxidation state assignments again depend on the carbyne type. For example, the Fischer carbyne, Br(Co)4WsCR, is considered as W(II) and the Schrock carbyne, Br3L2WsCRe, as W(IV). Once again, we have ambiguity in intennediate ca.ses. 

To Fischer carbenes correspond Fischer carbynes that are 18-electron complexes of electrophilic carbyne, and to Schrock carbenes correspond Schrock carbynes that are d° high-oxidation-state early transition-metal complexes of nucleophilic carbyne, often with a less-than-18-electron count. This latter category is important in alkyne metathesis, and all carbyne complexes show a rich chemistry. 

The carbyne ligand CR can be considered either as an LX ligand, or as an X3 ligand. In Fischer carbynes, the carbyne ligand is counted LX whereas in Schrock s carbynes, the carbyne ligand should be viewed as X3. This distinction follows the same logic as for metal-carbene complexes, i.e. is derived from the electronic strncture of the free carbyne ... 

A few simple examples should make this electron counting rule clearer. In Cr(CO)6, 16.17 [I I], the CO groups donate two a electrons each for a total of 12 electrons. The charge on the molecule and each CO is zero, so Cr is in the zero oxidation state, that is, it is a d complex (see 16.16). The total number of electrons associated with Cr is 6 + 12 = 18. In The Fischer carbyne complex [12], 16.18, the carbyne ligand is taken to be a cationic 2-electron cr donor (see 16.8) and Br is counted as an anionic two-electron donor (16.9). Together with the four phosphine... 

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