Cyclopentadienyl ligand compounds

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). 

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. 

Although allyltitanium compounds lacking cyclopentadienyl ligands are quite reactive, this does not affect their chemo-, regio-, or stereoselectivity. The ability of these reagents to discriminate between aldehydes and ketones has been tested in competition experiments. Notable examples are depicted in Table 13.1 [4,50a],... 

It has been reported that the steric requirements of the (ij5 -C5 Hs ) group are approximately equal to that of 2.5 carbonyl groups159, although again the homogeneous distribution of cyclopentadienyl ligands around a cluster is often limited. Actually, the compound Fe4(CO)4Cp4, which has the structure shown in Fig. 7, formally corresponds to the unknown Fe4(C0)i4. 

Cyclopentadienyl ligands have become extremely important in catalysis for metal such as Ti, Zr, and Hf (Chapter 10) and in academic studies of related elements such as Ta. Ethylene polymerisation with the use of CpiTiCE (alkylated with aluminium alkyl compounds) has been known for many decades, but the intensive interest in derivatives of these compounds started in the early 1980 s following the discovery of MAO (methaluminoxane - see chapter 10) which boosted metallocene catalyst activities by several orders of magnitude. Commercial interest focussed on ethylene copolymers (LLDPE where more homogeneous comonomer incorporation resulted in greatly improved copolymer properties) and in enantiospecific polymerisations for propene, styrene, etc. 

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

Although dicyclopentadienyllead(II) compounds, formally called plumbylenes, have been known since 1956, " they are not the congeners of carbenes since they are stabilized by r] -coordination of cyclopentadienyl ligands. In 1974, the first stable diaminoplumbylene [(Me3Si)2N]2Pb (173) was synthesized by Lappert and... 

The bonding of the three allylic carbon atoms to the metal in these compounds may be regarded in terms of a bidentate ligand. In the dimeric allyl-metal halides, this is consistent with the normal tetra-coordination, and in the cyclopentadienyl-allyl compounds it is interesting to note that the metal atoms are effectively penta-coordinated. Diamagnetism is preserved throughout. 

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