Metal-carbene complexes structure

Metal bromides, 4 322-330 Metal can food packaging, 18 37-39 Metal-carbene complexes, 26 926 Metal-carbon compounds, 4 648, 650 Metal-carbon eutectic fixed points, 24 454 Metal carbonyl catalysts, supported, 16 75 Metal carbonyl complexes, 16 73 Metal carbonyls, 15 570 16 58-78 bonding and structure of, 16 59-64 from carbon monoxide, 5 12 in catalysis, 16 72-75 economic aspects of, 16 71 health and safety aspects of, 16 71 heteronuclear, 16 69-71 high nuclearity, 16 66-69 high nuclearity carbonyl clusters, 16 64-66... 

Reaction of (butadiene)ZrCp2 (31/32), and substituted Cp variants, with a wide range of metal-carbonyl complexes, generates the chelated metal-carbene complexes 163 (equation 22)163. The crystal structure of a number of these complexes has been determined... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

The structure and bonding of metal silylene complexes varied from those of their carbon analogs. While Fischer-type metal carbene complexes without solvent adducts have been extensively characterized,48 most metal silylenoid complexes contain a bound solvent molecule or counterion on the silicon atom. The bond energy for donor silicon complex 22 was determined to be between 15 and 20kcal/mol 49,50... 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

Synthesis of transition metal carbene complexes is achieved by the silver(I) oxide method and subsequent transfer to the next transition metal (in this case gold) (see Figure 3.62). Two different types of silver(l) carbene complexes are realised, the monocarbene and bis-carbene complexes. Transfer of the carbene to a gold(l) centre does not always occur with retention of structure as only the gold(l) monocarbene complexes are observed. 

Once the hydroxy functionalised imidazolium salt is formed, it can be deprotonised and reacted with various metal complexes to form (transition) metal carbene complexes. The hydroxy group ensures that the ligand can be coordinated even to metals that are normally reluctant to form stable carbene complexes. A good example is the deprotonation of a hydroxyethyl functionalised imidazolium salt with potassium hydride [36]. The potassium cation coordinates to the oxygen atom of the alkoxide sidechain and forms cubes as structural elements (see Figure 4.6). The carbene end then coordinates to the respective... 

Metal-carbene complexes undergo numerous reactions, several of which are useful in the synthesis of complex organic molecules. These complexes are also intermediates in processes such as metathesis and ring-opening metathesis polymerization (Chapter 11). In Chapter 10 we will discuss the structure, synthesis, and... 

The picture is quite different for nucleophilic metal-carbene complexes. Here, contributing structures 10 and 13 seem to make the most contribution to the overall structure. Support for this observation comes from temperature-dependent NMR measurements8 of the M-C rotational barriers of various Ta-carbene complexes. The values obtained range from 12 to 21 kcal/mol, and seem to indicate considerable double bond character (structure 10). 

Mechanism 3 shows a pathway that was strongly influenced by the results of Herisson and Chauvin and is outlined in Scheme 11.2. Two key intermediates in this pathway are an alkene-metal carbene complex (5) and a metallacyclobu-tane (6), formed through concerted cycloaddition of the M=C and C=C bonds. A highly significant feature of the mechanism, caused by the unsymmetrical structure of 6, is its explanation of randomization early in the course of reaction. The Herisson-Chauvin mechanism does not require a specific pair of alkenes to interact directly for metathesis to occur, hence the name non-pairwise mechanism. 

The metal carbene complex [Mt3 CHP j has the formal structure 2 where n denotes the number of monomer units already added. 

Thus the synthesis, detailed structural analysis and reactivity of complexes II and III make an important contribution to the mechanisms suggested previously for the metathesis of olefins and for the polymerization of acetylenes by metal-carbene complexes . 

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