Carbenes and Carbynes

Mononuclear Carbenes and Carbynes.—The osmium-carbyne complex (1) has been structurally characterized, and the Os=C distance found to be 1.77(2) A and the Os-C -C angle 164(2)°. The osmium-carbon triple bond in (1) interacts in an acetylene-like manner with AgCl to give the dimetallocyclopropene derivative (2), in which the Os-Q distance has increased to 1.839(5) A. 

Hydride addition in the cationic osmium carbyne complex [Os(CC6H4-pMe)-(CO)2(PPh3)2]+ unexpectedly occurs at the / ara-position of the aromatic ring of the carbyne ligand, giving the unusual vinylidene complex (3).  

Protonation of the electron-rich complex ( -C5H5)Fe(dppe)(CH2CN) is accompanied by an unusual intramolecular rearrangement to give the new car- 

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A variety of attempts has been made to model the single steps of the Fischer Tropsch reaction on a molecular level. Naturally, the question of the catalytic activity of intermediate carbene and carbyne complexes has been pursued [4],... 

Carbene and Carbyne Complexes of Ruthenium, Osmium, and Iridium... 

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 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. 

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. 

Carbyne complex chemistry of osmium and ruthenium is discussed in this section. These studies demonstrate clearly the parallels that exist between the metal-carbon bonds in carbene and carbyne complexes and again emphasize the importance of metal basicity in determining complex reactivity. 

The similarity between the bonding models for transition metal carbene and carbyne complexes was noted in Section II. That the reactivity of the metal-carbon double and triple bonds in isoelectronic carbene and carbyne complexes should be comparable, then, is not surprising. In this section, the familiar relationship between metal-carbon bond reactivity and metal electron density is examined for Ru and Os carbyne complexes. 

The issue of the chemical nature of graphene edges, and thus of (re)active sites, was addressed head-on by Radovic and Bockrath [29]. Density functional theory was used to minimize problems that truly ab initio approaches such as Hartree-Fock are known to have, but careful comparison with experimental results in several seemingly unrelated fields was also performed. The key argument of relevance here is that, rather than being H-terminated, the sites that are most relevant for the chemical (re)activity of sp2 hybridized carbon materials are of carbene- and carbyne-type, as illustrated below for the zigzag (a) and armchair edges (b). 

Fig. 3.30. Preparation of carbene complexes by sequential [2 + 2] cycloadditions and [2 + 2] cycloreversions of carbene and carbyne complexes to alkenes and alkynes [596-598].

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