Carbene and Carbyne Complexes of Groups

E. O. Fischer, F. J. Gammel, J. O. Besenhard, and A. Frank, and D. Neugebauer, J. Organomet. Chem., 1980, 191, 261. Transition-metal carbyne complexes LIV. New metallocene-carbene and carbyne complexes of Group VI, preparation and electrochemical characteristics. 

A number of group VI carbonyl silyl substituted carbene and carbyne complexes have been reported114. The authors concluded that significant n interactions occur between the metal and the silicon atom either by hyperconjugation or participation of the silicon d orbitals. The chemical shifts for the carbene and carbyne complexes of the same metal were similar. For example, the chemical shfts of (CO)5W = C(OMe)(SiPh3) and (C5H5)(CO)2W=CSiPh3 are —18.3 and —26.7 ppm, respectively. 

Carbene and Carbyne Complexes of the Group VIII Metals... 

The format of this chapter will follow that used in previous volumes. The first section will deal with any review articles that are of relevance to this chapter. Section 2 will focus on articles relating to metal-earbon a-bonds involving Group 8, 9 and 10 metals, whilst Section 3 will be eoneerned with the carbene and carbyne complexes of those metals. 

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. 

The similarity, then, between carbene and carbyne complex chemistry of Group 8a transition metals, as well as of Group 6a and 7a metals, is apparent. 

Heavier Group 14 Analogs of Carbene and Carbyne Complexes... 

Electrophilic reagents react not only at the a-carbon of alkyl groups, but at the a-carbon of carbene and carbyne complexes. In these cases, the electrophile can either form a stable adduct by coordination to the nucleophilic carbon, or it can abstract a labile group. As noted in the section of Chapter 13 on carbene complexes, nucleophilic early metal alkylidene complexes, such as Cp HCH, coordinate Lewis acids at the carbene carbon. CpjTiCHj coordinates Me AlCl to form Tebbe s reagent. 

The discussed reactions of carbene and carbyne complexes show that they have essential significance as catalysts or unstable transient intermediate compounds in such catalytic processes as metathesis of olefins and other unsaturated compounds, Fischer-Tropsch synthesis, syntheses of cyclopropanes from diazoalkanes and olefins, and polymerization of olefins and alkynes as well as in organic synthesis. Except for alkynes [reaction (5.132) ] some compounds containing double bonds react with carbon monoxide and carbene ligands to form bonds with those groups. Examples of such compounds are enamines, ynamines, and Schiff bases. The JV-vinylpyrrolidone (enamine), methoxyphenylcarbene, and excess of CO (higher pressure) react to furnish enaminoketone. 

The running order in this chapter is essentially similar to that used in the preceding one. The main section, dealing with metal-carbon Group VIII triads in the order, iron, cobalt, and nickel, and is followed by a shorter section on carbene and carbyne complexes. Oxidative addition or reductive elimination reactions are included only when they lead directly to the formation or rupture of metal-carbon bonds. 

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. 

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. 

Complexes Containing Metal-Carbon a-Bonds of the Groups Scandium to Manganese, Including Carbenes and Carbynes ... 

The general format of this chapter is the same as that used in previous years. The review is restricted to detectable complexes of Groups 8,9, and 10 which contain a metal-carbon o-bond, including carbenes and carbynes. Species postulated as intermediates (e.g. in catalytic cycles) have been in the main excluded, and coverage of metal-metal bonded complexes has been minimised. 

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