Carbene/carbyne complexes compounds

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 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 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. 

More recently, Schrock has reported the formation of coordinatively unsaturated Ta and W carbyne complexes (124). Like unsaturated carbene complexes, these carbyne compounds are now established as being active intermediates in a number of catalytic reactions. The discovery of acetylene metathesis reactions catalyzed by carbyne complexes (3), for example, has generated considerable interest in this class of compound. 

Alkoxy carbene complexes are useful starting compounds for other organometallic complexes,1 5 particularly methylidyne (carbyne) complexes.16 By modification of the coordinated (noncarbene) ligands or of the carbene ligand, other carbene complexes can be synthesized. The use of carbene complexes in organic syntheses has been reviewed recently.17 18... 

The main synthetic technique for obtaining carbyne complexes is through the reaction of carbene compounds with metal halides, preferably with those of Group IIIA [259c] as, for example, through reaction (2.2) ... 

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. 

Scheme 8.12 Preparation of the first bis(carbene) tantalum complexes 46 and 48 from mono(carbene) or mono(carbyne) tantalum compounds as starting materials...

Following the Fischer procedure, alkynyl carbyne complexes trans-X(CO)4M=C—CPh 189 have been obtained in 30-60% yields by reaction of (l-alkynyl)carbene complexes la,b (M = Cr, W) with BX3 (X = Cl, Br, I). To date, (l-alkynyl)carbyne compounds have found application as catalysts as well as stochiometric reagents in organic synthesis.205c-206 Among the transformations reported thus far is the formation of a 4-amino-l-metalla-l-yne-3-ene (= enamino carbyne complex) 190 by addition of HNMe2 to compound 189 (Scheme 79).207... 

In practice this approach to the synthesis of compounds with dimetallacyclo-propane or -propene ring systems has been very successful. Complexes (1) - (12) are representative of those which have been readily prepared by reacting various low-valent metal compounds either with carbene-metal species, or with the carbyne complexes [w=CR( CO) 2(11-05 )] or [w=CR(Br)(CO) ]... 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

In connection with carbene and carbyne complexes, an especially interesting tungsten compound is 18-C-XXV, which contains W=C, W=C, and W—C bonds, whose lengths are 1.79, 1.94, and 2.26 A, respectively. 

In this chapter, we have considered just a few types of reactions of organometallic compounds, principally the replacement of CO by other ligands and the reactions involved in syntheses of carbene and carbyne complexes. Additional types of reactions will be discussed in Chapter 14. We conclude this chapter with two examples of how spectral data may be used in the characterization of organometallic compounds. Further examples can be found in the problems at the end of this chapter and in Chapter 14. 

Ernst Otto Fischer (bom 1918) was a student of Walter Hieber at Technische Hochschule Miinchen where he received his Ph.D. degree in 1948. He elucidated the molecular stmcture of ferrocene shortly after this compound was discovered. Further highlights in his life s work were the synthesis of dibenzene-chromium (C6H5)2Cr in 1955, the discoveries of the first metal carbene (1967), and the first me-tal-carbyne complex (1971). In 1964 he succeeded Walter Hieber to the chair of inorganic chemistry at Technische Hochschule Miinchen from which he retired in 1985. He received the Nobel prize for chemistry jointly with Geoffrey Wilkinson (Imperial College London) in 1973. 

Carbyne complexes can be synthesized in a variety of ways in addition to Lewis acid attack on carbenes, as described previously. Synthetic routes for car-bynes and the reactions of these compounds have been reviewed in the literature.17 Reactions of carbynes will be discussed in more detail in Chapter 10. 

The yellow, diamagnetic compound is thermally rather stable. Reaction of the carbyne complex in EI2O with NaOCy (Cy = cyclohexyl) gives the carbene complex [(C5Me5)(CO)2Tc=C(OCy)PhJ° ... 

The chemistry of metal carbene and carbyne complexes has undergone very rapid development since the sixties and olefin metathesis reactions now form part of a much larger family of [2 + 2] metathesis reactions between multiply bonded compounds. In Section 4.2 we shall briefly consider this wider context. Some examples will be given of the use of stoichiometric metathesis reactions of such complexes in organic synthesis (Dotz 1984 Schubert 1989). 

This section deals with the NMR studies of metal-carbene-carbyne, -isocyanide, and -carbonyl complexes. In these compounds the metal to carbon bond may have, at least formally, multiple bond character. The term synergic, or synergistic, is used to imply that there may be some variation of metal-carbon a bonding as a function of metal-carbon n bonding. 

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