Transition Metal Carbyne Complexes

The and NMR chemical shifts of coordinated carbenes are distinctive. The C chemical shifts of the carbene carbon in Fischer carbene complexes with oxygen donors on the carbene resonate between 290 and 365 ppm versus tetramethylsilane. Those with nitrogen donors resonate less far downfield, but generally give rise to signals between 185 and 280 ppm. The chemical shifts of the carbene carbons in Schrock carbene complexes are also far downfield. These carbons t5 ically resonate between 240 and 330 ppm. The H NMR chemical shifts of these species are also far downfield and are typically found between 10 and 20 ppm. The infrared vibrations are difficult to locate because the M=C bond vibrates at a low frequency, and these bands are generally not identified. 

Orbital symmetries involved in the bonding of an alkylidyne ligand. 

X-ray diffraction data of a complex with a carbyne, carbene, and alkyl group and two carbyne complexes with different substituents. 

I hope that I have been able to demonstrate, with this small selection of our newest research results, what a variety of reaction possibilities the chemistry of transition metal carbene complexes display. In the following I review an area whose development we have made most recently our special task, namely that of transition metal-carbyne complexes. 

The IR spectra indicated the presence of disubstituted hexacarbonyl-metal derivatives with two different ligands in trans positions (trans-(CO MR R2). Moreover, the cryoscopic molecular weight determination proved that the complexes must be monomers. 

Together with further spectroscopic findings, especially from 13C and TO NMR studies, this could only be interpreted to mean that besides the four CO ligands a halogen atom and a CR group are bonded to the metal (Fig. 8). 

For this new type of compounds we would like to propose the name carbyne complexes for two reasons (i) in analogy with carbene complexes, (ii) because similarly to alkyne, on the basis of the diamagnetism of these compounds, a formal metal-carbon triple bond had also to be postulated. 

The proposed triple bond should have a very short distance between the metal and the carbyne carbon. Only X-ray structural analyses could clarify this question as well as provide definitive confirmation of our structural proposal, and these studies were then carried out in our Institute by Huttner and his co-workers on three carbyne complexes. 

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H. Fi.scher, C. Troll, and J. Schleu, in Transition Metal Carbyne Complexes, (F. R. Kreissl ed.), p.79. Kluwer Academic Publishers, Dodrecht, 1993. 

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

The chemistry of transition metal-carbyne complexes is rather less developed than the chemistry of carbene complexes. This is almost certainly because reactions which form new carbyne complexes are relatively rare when compared with those forming metal carbenes. The few theoretical studies of carbyne complexes which are available indicate that close parallels exist between the bonding in carbene and carbyne compounds. These parallels also extend to chemical reactivity, and studies of Group 8 complexes again prove instructive. 

Transition metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds has proved generally applicable. In addition to making the initial characterization (723), the Fischer group has made the largest contribution to carbyne complex chemistry, with some 200 mononuclear complexes of Group 6 and 7 metals having been prepared. 

There are, broadly speaking, three general routes to transition metal carbyne complexes ... 

Table 7.10 Calculated BDEs De (Do) (kcal/mol) of transition metal carbyne complexes.3...

Transition metal carbyne complexes are described by the general formula L M=CR where the carbyne ligand (=CR) is bonded to the metal by a metal-carbon triple bond. Transition metal carbene complexes have found numerous applications in synthetic organic chemistry through a variety of carbene transfer and cycloaddition reactions [17]. In contrast, carbyne (L M=CR) and vinylidene (L M=C=CRR ) complexes have far fewer applications, in part because their overall chemistry is significantly less developed [18]. Addition reactions to transition metal vinylidene complexes will be discussed in Chapter 21. The first successful synthesis of a carbyne complex was reported by Fischer and co-workers in 1973 [Eq. (8) 19]. Subsequently, many other carbyne complexes have been synthesized by the classic route of Fischer or by new synthetic methods [20]. 

A potential source of an X substrate is, of course, an X = X type of molecule. Reaction 6 would then simply represent a metathesis reaction. Since transition metal carbyne complexes are well known, it is not inconceivable that Reaction 7 can occur. Alternatively an X = X or... 

Alkynes can also undergo metathesis reactions catalyzed by transition-metal carbyne complexes. The intermediates in these reactions are believed to be metallacyclobutadiene species, formed from the addition of an alkyne across a metal-carbon triple bond of the carbyne (Figure 14.35). The structures of a variety of metallacyclobutadiene complexes have been determined, and some have been shown to catalyze alkyne metathesis. 

Kreissl, F. R. (Ed.). Transition Metal Carbyne Complexes, Proceedings of the NATO Advanced Workshop on Transition Metal Carbyne Complexes, Wildbad, Kreuth, Germany, 27 September-2 October 1992. NATO ASI Ser., Ser. C, 1993, 392. 

Weiss, K., Goller, R., Denzner, M., LdBel, G. Kddel J. (1993) in Transition Metal Carbyne Complexes, (Hrsg. F.R. KreiBl), Kluwer, Dordrecht, 55. 

Weiss, K. (1988) Catalytic Reactions of Carbyne Complexes in H. Fischer, Transition Metal Carbyne Complexes, VCH Publisher Weinheim. 

F. R. Kreifil (ed.). Transition Metal Carbyne Complexes, 23—37. 1993 Kluwer Academic Publishers. 

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