Metal-carbon triple bond

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) ... 

From Carbyne Complexes. Addition of HC1 across the metal-carbon triple bonds of Ru and Os d8 arylcarbyne complexes yield stable, neutral secondary alkylidene complexes ... 

Reactions of carbyne complexes that maintain the integrity of the metal-carbon triple bond form the third route to new carbynes. Substituent modification, ligand exchange, oxidation, and reduction reactions have all been reported (see, e.g., Ref. 126). 

A decade after the announcement of the metal-carbon double bond, Fischer s group reported the first complex containing a metal-carbon triple bond.711... 

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. 

Section III of the review is concerned with alkylidene and alkylidyne halocarbonyl complexes, and a review on recent advances in the chemistry of metal-carbon triple bonds by Mayr and Hoffmeister in 1991 contains material of interest.6 Section IV is concerned with halocarbonyl alkyne and alkene complexes described from 1987 to mid-1995, because an exten-... 

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. 

Despite Schrock s extensive work on tantalum, molybdenum, and tungsten car-byne complexes (e.g., 47, 49, 50, 51, and 52) [96, 97, 99, 100], it must be recalled, however, that they were not the first compounds with a metal—carbon triple bond. 

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

G. Huttner, H. Lorenz, and W. Gartzke, Transition Metal-Carbon Triple Bonds, Angew. Chem. Int. Ed. Engl. 13, 609-610 (1974). 

A. Mayr, and H. Hoffmeister, Recent Advances in the Chemistry of Metal-Carbon Triple Bonds, Adv. Organomet. Chem. 32, 227-324 (1991). 

In addition to complexes of terminal alkylidenes, many complexes containing alkylidenes bridging two metal atoms have been made these are commonly known as p-methylene complexes. A wide range of complexes containing other metal-carbon bonds are known such as the carbyne complexes (for example, Cr = CPh(CO)5) which contain metal-carbon triple bonds. While such species may well be involved in some catalytic reactions, their importance in industrially catalyzed processes still has to be evaluated. 

Carbyne complexes have metal-carbon triple bonds they are formally analogous to alkynes. Many carbyne complexes are now known examples of carbyne ligands include the following ... 

IUPAC has recommended that alkylidyne be used to designate complexes containing metal-carbon triple bonds. 

Alkynes can also undergo metathesis reactions catalyzed by transition metal car-byne complexes. The intermediates in these reactions are believed to be metallacy-clobutadiene species, formed from the addition of an alkyne across a metal-carbon triple bond of the carbyne (Figure 14-26). The structures of a variety of metallacyclobu-tadiene complexes have been determined, and some have been shown to catalyze alkyne metathesis. 

Nucleophilic displacement at a terminal methylidyne ligand, with regeneration of a metal - carbon triple bond in 199 (E = S or Se, R = Me, Ph, or P-NO2QH4) to form a new methylidyne ligand, occurs in the reaction of 198 [L = HB(3,5-Mc2—C3HN2)3] with thiolate or selenolate anions under phase-transfer conditions 149). 

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). 

Aristov and Armentrout (25) and Freiser ct al. (26.27) derived the bond energies of metal-carbon triple bonds from gas-phase experiments. The values are listed in Table III. Table 111 also lists bond energies for metal-carbon double and single bonds. 

Substitution of the ligand trans to the metal-carbon triple bond, usually a halide, was observed in several situations. Substitution of halide in the tetracarbonyl systems is rare because most nucleophiles will substitute carbon monoxide. It was found that the halide is substituted by the carbonyl metallate anions [Mo( / -CjH5)(CO)3]" and [Co(CO)4] [Eq. (74)]... 

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