Ligands cyclopentadienyl

The monomeric iron complexes ( / -cp)FeX(CO)2 (X = Cl, Br) undergo photoinduced loss of halide ion  

The reaction is carried out in a solvent with a high dielectric constant such as DMSO, and the final product is the dimer [( -cp)Fe(CO)2)]2. No formation of free halogen is detected, and isobestic points are observed in the electronic absorption spectrum when the photolysis is carried out at wavelengths 400 nm. Photolysis 

The photolability depends on the particular ligands in the complex. An example that illustrates this point is the pair of complexes ( / -cp)Mn(CO)2py and ( / -cp)Mn(CO)2PPh3. For the case of ( / -cp)Mn(CO)2py, the pyridine ligand is preferentially photolabilized, but for the complex ( / -cp)Mn(CO)2PPh3 it is the carbonyl ligand that is photoabilized  

The photolysis of [// -cpFe(CO)2]2 in the presence of ligands (L) results in substitution. The reaction can proceed via homolysis of the metal-metal bond to give 17-electron radicals (L = P(OMe)3), or via the intermediate complex (rj -cp)2Fe2(//-CO)3/ An example is shown in Eq. (6.59) for L=MeCN. Asimilar chemistry is observed with [ / -cpMo(CO)3]2/  

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Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

Molecular examples of trivalent molybdenum are known in mononuclear, dinuclear, and tetranuclear complexes, as illustrated in Figure 5. The hexachloride ion, MoCk (Fig- 5a) is generated by the electrolysis of Mo(VI) in concentrated HCl. Hydrolysis of MoCP in acid gives the hexaaquamolybdenum(III) ion, Mo(H20) g, which is obtainable in solution of poorly coordinating acids, such as triflic acid (17). Several molybdenum(III) organometaUic compounds are known. These contain a single cyclopentadienyl ligand (Cp) attached to Mo (Fig. 5d) (27). 

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

Metallocene (Section 14.14) A transition metal complex that bears a cyclopentadienyl ligand. 

Control experiments, performed with the ( + )-(R)-diastereomer of 1, which differs only in the configuration of the stereogenic center at the metal, afford the enantiomeric homoallylic alcohol, (S)-3-methyl-1-phenyl-3-butenol, also with high enantiomeric excess, indicating that the chiral cyclopentadienyl ligand has no dominating influence1-2. 

Optically Active Ring-Annulated Cyclopentadienyl Ligands. . . . 135... 

The unique importance of the cyclopentadienyl ligand in the coordination chemistry of transition metals is mainly based on following three features ... 

The scope of the present paper is limited to those cyclopentadienyl ligands that contain more than two bulky substituents and transition metal complexes derived thereof in order to be able to focus on the specific effects of these ligand systems. A selection of some mono-substituted cyclopentadienyl ligands will be treated also. Among the numerous reviews highlighting special aspects of cyclopentadienyl... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

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