Carbene bonding

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

Ru—C(carbene) bond distances are shorter than Ru—P bond lengths, but this can simply be explained by the difference in covalent radii between P and The variation of Ru—C(carbene) bond distances among ruthenium carbene complexes illustrates that nucleophilic carbene ligands are better donors when alkyl, instead of aryl, groups are present, with the exception of 6. This anomaly can be explained on the basis of large steric demands of the adamantyl groups on the imidazole framework which hinder the carbene lone pair overlap with metal orbitals. Comparison of the Ru—C(carbene) bond distances among the aryl-substituted carbenes show... 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

R = Pr) via a bromide displacement process. Halide displacements have been observed previously in the reactions of carbenes with Me3SiI (38). However, this represents the first such reaction with a haloborane. The X-ray crystal structure of 49 was determined and showed that both heterocyclic rings are planar and that the interpla-nar angle is 92.9°. The B-C(carbene) bond distance of 1.580(11) A is comparable to that found in 32 (1.603(3) A). 

The diazo-compounds and corresponding aromatic carbenes that form the basis for our dissection of structure and reactivity are shown in Table 1. The carbenes in this group are carefully chosen so that the variation in structure is systematic the theory identifies the carbene bond angle and certain electronic factors as controlling chemical and physical properties, and as far as possible, these two features are varied independently of each other for these carbenes. Table 2 lists some other aromatic carbenes that have been studied. In general, the structures of these carbenes are not simply related to each other. Nevertheless, the principles uncovered by analysis of the compounds of Table 1 can be readily extended to those of Table 2. 

The metal-carbene bond distances in this family of complexes (2.082 (2) A for Ag, 1.9124 (16) A for Cu, and 2.035 (12) A for Au) are within the range of reported values for typical group 11 metal NHC complexes (23). The metal carbene units are almost linear, with a C-M-C bond angle of 178.56 (13)°, 177.70 (9)°, and 177.7 (6)° for Ag, Cu, and Au, respectively. The imidazole units for 2 -Ag, 2 -Cu, 2Me Au exhibit structural parameters typically observed for coordinated NHC ligands. There are no inter- or intramolecular metal-metal interactions in these complexes. 

In summary, the metal-carbon bond of an NHC is significantly different from a real metal-carbene bond both of the Fischer- or Schwck-type. Thus, the representations of the metal-carbon bond according to Scheme 9 is now generally accepted. 

Propagation follows in a similar manner (Eq. 7-97), and the overall result is the insertion of the halves of the double bond of monomer into the metal-carbene bond. Evidence for... 

Figure 8.1. Relationship between the carbene bond angle and the nature of the frontier orbitals.

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