Linkages metal-olefin

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

Anderson showed that ethylene could be displaced from Zeise s salt by various other ligands (5). However, the nature of the olefin-metal linkage was puzzling. Winstein and Lucas (282) proposed that the linkage in olefin-silver complexes involved resonance between three canonical forms ... 

Similarly, the adsorbed state of 1,2-dienes may be represented in two ways, as shown in Structures (IV) and (V). Since the two 7r-electron systems in a 1,2-diene are mutally at right angles it follows that the two metal-olefin bonds in Structure (V) must also be at right angles to each other. The fission of one olefinic linkage and the subsequent... 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - - ... 

Although transition metal-mediated P-H addition across ordinary alkenes proceeds well only with five-membered cyclic hydrogen phosphonates, addition across the olefinic linkage of a,P-unsaturated compounds occurs readily with a range of phosphorus species and catalytic agents. Of particular note are the reaction systems involving platinum,96-107 palladium,108-115 and the lanthanides.116-122... 

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Hengge, E. Properties and Preparations of Si-Si Linkages. 51, 1-127 (1974). Henrici-OHve, G., and Olive, S. Olefin Insertion in Transition Metal Catalysis. 67, 107-127 (1976). 

The stoichiometric reaction of CFC-113 with metallic zinc to produce chlorotrifluoroethylene is a well-known process. Catalytic approaches to this synthesis are also available. Charcoal (47) as well as supported nickel, iron, cobalt, and chromium operating at elevated temperature (48) give chlorotrifluoroethylene yields in excess of 90%. The selective replacement of the vinylic chlorine by hydrogen while still maintaining the olefinic linkage provides still another entry into trifluoroethylene. 

These mechanistic possibilities require further investigation. Some examples of how the reaction is influenced by the structure of the olefin, the catalyst, and the solvent are given. In the presence of MX(CO)(PPh3)2 (M = Rh, Ir X = Cl, I), the oxidation of a-and /3-methylstyrenes and cis- and frans-stilbenes permitted the establishment of an activity sequence dependent on the catalyst and the stucture of the olefin. The solvent effect has been studied in the Ru complex-catalyzed oxidation of styrene and methylstyrene. In the presence of a Rh or Ir complex, the oxidation of tetramethylthylene is very selective" and takes place at a faster rate than those of the less-substituted olefins. It emerges from this that a significant role is not played by the coordinative linkage between the metal center and the olefin. Examinations have also been made on the oxidation activities of metallo-porphyrins, for example, the oxidation of cyclohexene with Co and Rh porphyrins. ... 

A second source of evidence of the adsorbed state lies in the manner in which adsorbed 1,3-dienes react. This will be discussed fully in Section III, F, 6 it is sufficient for present purposes to state that, at the surfaces of most metals, one olefinic linkage appears to hydrogenate independently of the other to give adsorbed 1-butene. 1,2-Addition is thus generally preferred to 1,4-addition. The olefinic linkages retain their identity to a larger degree in Structure (II) than in Structure (I) and thus preferential 1,2-addition is more easily understood if the second structure is accepted. On the other hand, 1,4-addition would be expected to be at least half as important as 1,2-addition if Structure (I) was correct. It appears, therefore, that Structure (II) represents a preferable notation, based on the evidence at present available. 

The results of a polarographic and potentiometric study 42) on the affinity of several unsaturated carboxylic acids for various metal ions showed a much greater tendency for bonding through an olefinic linkage for Cu(I) as compared to Cu(II), although in some cases Cu(II) formed more stable complexes than... 

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