Metal-ligand bonds carbene complexes

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Dimetallocycles have been discovered which exhibit high reactivity with respect to carbon-carbon bond-making and -breaking processes. They allow the synthesis of a variety of simple but important hydrocarbon ligands bridging a dinuclear metal centre. y-Carbene complexes are readily available by several routes and their reactions have implications for both alkyne polymerisation and alkene metathesis. A substantial chemistry of organic species co-ordinated at dinuclear metal centres is in prospect, with significance for metal surface chemistry and catalysis. 

The bonding of a triplet carbene is less closely related to the metal-ligand bonds discussed thus far. The carbenes in these complexes are considered dianionic. With this assignment of electrons and charges, the a- and ir-orbitals of this dianionic fragment can be considered to bind the metal by donation of two electron pairs, one into each of the unoccupied orbitals of the metal of a- and ir-symmetry. These carbene ligands have ir-bonds that are stronger than those in Fischer carbenes, but much weaker than those in alkenes. The barrier to rotation in these complexes is t5q)ically 19 kcal/mol. - ... 

Following our interest on the redox properties of transition metal isocyanide and carbene complexes [1], we report the investigation of the electrochemical behaviour of new phosphonium-fiinctionalized isocyanide (A), and derived carbene (B), indole (C) and protonated indole (D) complexes of Cr, Mo and W pentacarbonyls. These studies appear to have been undertaken for the Erst time for complexes with such types of ligands. It was also our object to correlate the redox properties of these compounds with the electron donor/acceptor ability of these ligands. Moreover, this study would also extend to novel carbene complexes the rather limited electrochemical investigation reported [2] for compounds with multiple metal-carbon bonds. 

TM compounds in high and low oxidation states with metal-carbon double and triple bonds have been the subject of a systematic theoretical study. Table 7 shows the calculated metal-ligand bond lengths of several tungsten carbene and carbyne complexes at the HF and MP2 levels of theory. The HF optimizations were carried out using two different basis sets. Basis set I has DZ quality at tungsten (no additional 6p... 

Figure 30 (a), (6) and (c) gives representations of the bonding in valence bond terms, but these are rather limited descriptions. Such descriptions are useful in emphasizing the relative importance of the contributions to the metal-ligand bond of a-bonding (a) and w-bonding (A). This problem of representation is further illustrated by the rather extreme example of cyclopropene, which is usually written as 5.5, but an alternative representation is 5.6, where the molecule is shown as a w-acetylene complex of carbene. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Free carbenes can also be avoided by using transition metal-carbene complexes L M—CRR (L = a ligand, M = a metal),which add the group CRR to double bonds.An example is ... 

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... 

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