Ferrocene, metallocen ligands

In recent years we have developed a new type of planar-chiral ligand system which is based on a phosphaferrocene skeleton equipped with an additional donor function Y [8]. This structure is closely related to the well-known ferrocene-type ligands in that a CH unit of the latter has been replaced by a P atom. The phosphaferrocene moiety serves as both a chiral metallocene-type backbone and as a donor group via the phosphorus atom lone pair. These new ligands are unique in their topological architecture and show interesting ligand properties. 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

The elements of stereoisomerism considered thus far consist of a point occupied by an atom and a line wholly occupied by one or more bonds. The ligands are joined directly to these elements. This emphasis on bonding relationships appears to be proper, as the distinction between constitutional and steric isomerism similarly depends on established patterns of connectedness. From this point of view it seems less than satisfactory if direct connectedness between specific atoms is assumed, when chemical theory envisions no such localized bond. This situation prevails in the description of ir complexes such as the metallocenes. Initially (44a), the 1,2- (22) or 1,3-heteroligated ferrocenes were considered to... 

METALLOCENES. A class of neutral transition meial compounds containing two eyclopeniadicnyl tC Ho ligands. 7 -bonded to a ccmral metal atom in a "sandwich" structure, as exemplified by ferrocene (sec belosvi. 

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