7t ligands

Formation and characterization of zirconacyclopropanes and zirconacyclopropenes In addition to / -H abstraction of dialkylzirconocenes discussed earlier (Schemes 5 and 6), several other methods are also available for the preparation of three-membered zirconacycles as summarized in Scheme 35. From the viewpoint of cyclic carbozirconation reactions, especially those under Zr-catalyzed conditions, /3-H abstraction, 7t-ligand substitution, and decarbometallative ring contraction are particularly important. As such, these three-membered zirconacycles are generally unstable, but they can be stabilized with phosphines, for example, PMe3, and other bases, and are fully identified. Some of the well-identified examples are shown in Scheme 36.206-209... 

The 7t ligands play important roles in a large number of homogeneous catalytic processes. Alkene polymerization and a variety of other reactions involve alkene coordination (see Chapters 6 and 7). As the name suggests, CO is the main ligand in carbonylation reactions (see Chapter 4). All four ligands CO, alkene, H , and PR3, play important parts in hydroformylation reactions (see Chapter 5). 

T-ligands framework. The most typical situations are shown in Figure 7. If the pre-catalyst has an overall Ci-symmetry, the two coordination positions are sterically and electronically different. The activity and catalytic properties of the two possible sites can be completely different. If the pre-catalyst has an overall -symmetry, the two coordination positions are homotopic and thus the activity and catalytic properties of the two sites are identical. If the pre-catalyst has an overall Cg-symmetry, two cases are possible. If the local mirror plane contains the coordination positions available to the growing chain and the monomer, the two coordination positions are not symmetry related, and, by symmetry, they are both non enantioselective. More interesting is the case in which the local mirror plane relates the coordination positions available to the growing chain and to the monomer. In fact, in this case the two coordination positions are enantiotopic, and thus the possible asymmetric induction (in the framework of the chain-migration mechanism) is opposite at each insertion step. 

Concerning the performance of ansa-Cp-amido complexes, many papers discuss the influence of the 7t-ligand type and substitution,453-457 bridge type,458-464 cr-ligand type and substituents,463 465 466 and the different metals.455... 

Electrophilic abstraction of a portion of a a-bound hydrocarbyl or a coordinated 7t ligand may occur. The trityl cation, Ph3C+, is a commonly-used electrophile for this task. Equation 8.73 provides an example of abstraction at the (3 position of an alkyl ligand to provide an rf-alkene complex,102 a useful route for the synthesis of these compounds. 

Various unsaturated cyclic 7t-ligands undergo, within a transition-metal complex, cycloaddition reactions and rearrangements with simultaneous formation of cyclopropane subunits. This is observed with cycloheptatriene ehromium and iron complexes such as 4 which give cycloaddition products, e.g. 

Figure 8.11 Immobilization of a Zr(IV) complex of general formula [(7t-ligand)ZrCl2] onto oxidized CNTs modified with MAO. (From ref. 97d, with permission from Elsevier.)...

Transition Metal Compounds. - Macchi et al. performed the first experimental electron density study of a 7t-ligand -coordinated to a metal atom.156 They considered this work as an experimental test of the Dewar-Chatt-Duncanson (DCD) bonding formalism and expected that their work would provide information concerning the 7r-complex versus the metallacycle dichotomy. The authors claim that the successful application of AIM to the experimentally determined p has been the most important step in the coupling of X-ray studies and theoretical chemistry. Their paper reports on the determination of an accurate electron density of crystalline bis(l,5-cyclooctadiene)-nickel, Ni(COD)2 (Figure 11) by X-ray diffraction at 125 K. 

Sometimes thermal and photochemical activation can result in the formation of different products. For example, heating the solution of the ethylrhodium carbonyl complex RhHBPz 3(CO)(C2H4) (Pz = 3,5-dimethylpyrazole) in benzene entails the elimination of the ethylene 7t-ligand and the formation of phenyl-rhodium hydride (complex IV-3 in Scheme IV. 11) [20a]. On irradiation of this solution, the hydride ligand adds not to the metal atom but to the ethylene molecule, which results in the appearance of an o-ethyl group in the complex IV-4 [20b]. 

A synthesis of [Ir(bipy)3] has been reported by Flynn and Demas." C and proton NMR spectra have confirmed the tris-complex assignment. [IrCbipylj] exhibits a phosphorescence at 22.3 kK with a mean lifetime of 80 /2s in MeOH/EtOH glass at 77 K luminescence was assigned as predominantly t 7t ligand phosphorescence. 

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