Metal Carbynes

Carbometallation is a term coined for describing chemical processes involving net addition of carbon-metal bonds to carbon-carbon Jt-bonds [1] (Scheme 4.1). It represents a class of insertion reactions. Whereas the term insertion per se does not imply anything chemical, the term carbometallation itself not only explicitly and clearly indicates carbon-metal bond addition but also is readily modifiable to generate many additional, more specific terms such as carboalumination, arylpalladation, and so on. In principle, carbometallation may involve addition of carbon-metal double and triple bonds, that is, carbene- and carbyne-metal bonds, as well as those of metallacycles. Inasmuch as alkene- and alkyne-metal Jt-complexes can also be represented as three-membered metallacycles, their ring expansion reactions via addition to alkenes and alkynes may also be viewed as carbometallation processes (Scheme 4.1). 

Kreis from 16 and BI3) [43] that the seminal paper about the synthesis and characterization of the first carbyne metal complex was submitted to Angewandte Chemie. When Fischer went to Stockholm in December 1973, he had a ball-and-stick model of Cr(CCH3)(CO)4l in his luggage, demonstrating to the audience of his Nobel Lecture that a transition metal compound with a metal-carbon triple bond is not a fantasy, but really exists. 

E. O. Fischer, U. Schubert, and H. Fischer, Selectivity and Specificity in Chemical Reactions of Carbene and Carbyne Metal Complexes, Pure Appl. Chem. 50, 857-870 (1978). 

A. Mayr, G. A. McDermott, and A. M. Domes, Synthesis of (Carbyne)metal Complexes by Oxide Abstraction from Acyl Ligands, Organometallics 4, 608-610 (1985). 

The chemistry of metal-carbon triple bonds has developed at an increasing pace since the discovery (7) of the first carbyne metal complexes by E. O. Fischer and co-workers in 1973 (2-4). Two extensive reviews on the field by Kim and Angelici (5) and by Fischer, Hofmann, Kreissl, Schrock, Schubert, and Weiss (6) have been published in recent years. In addition several articles focusing on the research areas of the respective authors have been published (7-19). The chemistry of metal-carbon triple bonds is also covered within the context of more general yearly literature surveys (20). 

In a study by Brower, Templeton, and Mingos the electronic properties of bis-oxo, oxo-carbyne, and bis-carbyne metal complexes were compared... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

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... 

Examples of carbyne-metal bonds in organometallic complexes, the adequate model being the framed one, a robust cluster... 

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

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

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... 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

BONDING MODELS AND REACTIVITY PATTERNS FOR TRANSITION METAL CARBENE AND CARBYNE COMPLEXES... 

The wealth of empirical information collected for transition metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Perhaps the most significant contribution made by the theoretical studies of carbene and carbyne complexes concerns an understanding of the reactivity patterns they display. In this section the relationship between bonding and reactivity is examined, with particular emphasis being given to the ways in which studies of Ru, Os, and Ir compounds have helped unify the bonding models applied to seemingly diverse types of carbene and carbyne complexes. 

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. 

In the cationic complex [(d75-C5H5)(CO)2Mn=CMe]+ the lack of symmetry in the metal fragment means that the two 7r-bonds are not degenerate. The percentage electron density of the two 7r-orbitals on the carbyne carbon have been calculated as 33 and 35%—relatively large figures which clearly support the triple bond formulation (28). 

The change to a silicon-based substituent group, e.g., SiMe3, has the opposite effect. The introduction of two more orbitals of 7r-symmetry appropriate for bonding stabilizes the metal-carbon interaction and increases the percentage electron density of the 7r-orbital on the carbyne ligand (28). 

Charge. The small amount of charge distribution data for carbyne complexes (based on Mulliken population analyses) indicates that the metal-carbon bond is generally polarized Ms+—C5- and that the carbyne carbon is always more negative than adjacent carbonyl carbons (28,30). These conclusions are directly analogous to those derived for carbene complexes. 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

Many anionic nucleophiles add to the carbyne carbon in cationic Fischer complexes, and these reactions can be used to synthesize carbene ligands unavailable by other routes. The stepwise reduction of a metal-carbon triple bond has been demonstrated (32) ... 

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