Cyclopentadienyl complexes with /-block metals

Recent efforts to explore the synthesis and reactivity of organotransition metal complexes with the cyclopentadienyl bisphosphine ruthenium auxiliary have emphasized the potential of this system for the development of new organic synthetic methods, but they have also uncovered a number of stumbling blocks which must be overcome to achieve real success in this area. Future developments in the chemistry of these ruthenium complexes will be based on the successful application of the reactivity trends uncovered in the past work. The rapid expansion of studies in this field since the mid-1980s has made it increasingly difficult for the synthetic chemist to... 

Novel rathenium complexes with carborane ligands were employed as efficient catalysts for controlled polymer synthesis via Atom Transfer Radical Polymerization (ATRP) mechanism. The ability of carborane ligands to stabihze high oxidation states of transition metals allows the proposed catalysts to be more active than their cyclopentadienyl counterparts. The proposed catalysts do not reqnire additives such as aluminium alkoxides. It was shown that introdnction of amine additives into the polymerization mixture leads to a dramatic increase of polymerization rate leaving polymerization controlled. The living nature of polymerization was proved via post-polymerization and synthesis of block copolymers. 

Cyclopentadienyl rings form diifferent compounds depending on the element linked to them. In some cases, a more or less deformed cyclopentadiene-like ring is involved in the complex systems of interactions with atoms (ions) other than carbon elements. The typically nonmetallic elements are bound via a covalent link C—X. Typical ionic interactions are encountered for compounds with s-block elements of the Periodic Table. Coordination compounds are formed mainly with transition metals. These kind of compounds are the most complex, since they are often built up of several ligands which are located in various places in the spectrochemical series and may... 

Unlike the case of many other cyclopentadienyls, these complexes have a particularly crowded ligation environment thus having no analogous structural counterparts in the d-block metals. Both the simpler M(C5H5>4 [27] and the ring substituted [28] M[C5H4(CH3)]4 (M = Th, U) complexes (hereafter M(cp)4 and M(cp )4) have been studied. Nevertheless, the superior resolution found with spectra of the M(cp >4 systems makes these complexes a better object of discussion. The complexes M(cp" )4 have ligation environments of tetrahedral symmetry [28]. 

The cyclopentadienyl group, C5H5, forms complexes with all the d-block transition elements. Its usual mode of bonding is fy , although there are examples of and f] attachment. In the following series of compounds the 18-electron configuration of the metal is maintained by variation of the number of electrons provided by one of the cyclopentadienyls (Fig. 9.1). 

The base-free compounds of CpjLn type are known for all metals of the being considered block including radioactive promethium (Table III.3.). The cyclopentadienyl complexes of trivalent Sc, Y, La, Ce, Pr, Nd, Sm and Gd prepared in 1954 by Wilkinson and Birmingham in the reaction of anhydrous metal chlorides with CpNa were the first REM organoderivatives [119]. Later this method has been modified and applied to Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu [88, 91, 120, 121]. Up to now it remains to be the main way to the tricyclopentadienyl REM complexes. Besides CpNa, cyclopentadienides or substituted cyclopentadienides of lithium and potassium are used in these reactions [31,95, 100,101, 111, 116] ... 

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. 

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