Metallocenes, paramagnetic

In principle it should be possible to determine the anisotropy of the paramagnetic susceptibility for some systems of pseudo-axial symmetry, but apart from the citation (74) of a private communication regarding the ferricenium cation, no experimental data are available for metal sandwich complexes. Such measurements should however be possible for at least some metallocenes and mixed sandwich complexes since these are found to crystallise in either a monoclinic (Fe(Cp)2 (6)) or orthorhombic (Ru(Cp)2, (Cp)V(Ch),... 

The usual carbon-) 3 chemical shift range of organic compounds ( 250 ppm) considerably expands in organometallic compounds. Paramagnetic metals in metallocenes (Table 4.72) induce particularly large 13C shift values [476]. 

Transition metal paramagnetic complexes that warrant further comment are the radical anions derived from metallocene- (especially ferrocene-) ketyls and related species, such as compounds IX (61), X (45), or XI (146), and metallonitroxides. From the ESR spectra of compounds... 

The molybdenum analog of chromocene see Metallocene Complexes) [CP2M0] (molybdenocene) is not stable and has only been isolated using matrix isolation techniques see Matrix Isolation). The IR, magnetic circular dichroism spectra of the matrix isolated [CP2M0] were interpreted in terms of a paramagnetic metallocene, with parallel rings. 

Modem transition metal organometallic chemistry can be said to have been invigorated by the discovery and subsequent bonding description of ferrocene (see Ferrocene) by Wilkinson and Fischer in 1951. As predicted by MO theory, all of the neutral first-row metallocenes (see Metallocene Complexes) (V Ni) are paramagnetic, with the exception of ferrocene. Metallocenes probably represent the most studied of all paramagnetic organometallic complexes. 

A few paramagnetic metallocene cations such as chro-mocinium (Cp 2Cr+) have been reported but are only stable when sterically protected. 

The analogous uranium(III) compounds also show unusual reactivity patterns. For example, addition of COT to (Cp )3U yields a mixed metallocene dimer (18) bridged by a COT ligand in this reaction, the (Cp )3U complex has effectively acted as a formal three-electron reductant (equation 3)." A variety of substituted (Cp )3U complexes form adducts with CO and CNR (isocyanides) - these are rare examples of actinide metals with r-acidic ligands." Paramagnetic uranium alkyl complexes are active in a range of catalytic processes." ... 

The synthesis of paramagnetic materials that have specifically tailored magnetic properties is gaining considerable interest (see Magnetism of Extended Arrays in Inorganic Solids and Magnetism of Transition Metal Ions). For example, metallocenes and their derivatives are known to... 

A wide variety of paramagnetic organometallic compounds are known. The large majority of these are classified as metallocenes and are of considerable NMR interest. In studying such systems Kohler (179) has suggested that spectroscopy offers distinct advantages over spectroscopy. From studies on bis-(h -t-butyl-... 

The carborane analogues of the metallocenes, the so-called metallocarboranes, have been studied by Wiersema and Hawthorne. (193) The paramagnetic species that they chose to study are of the types (C5H5)M(C2B H + 2) and M(C2B H +2)2 where M = Cr(iii), F iii), Ni(iii), and Co(ii), and a = 9,8,7, and 6 (Fig. 4). The B isotropic shifts of the Fe(iii) and Cr(iii) complexes reflect large negative hyperfine coupling constants which are consistent with a 7r-polarization mechanism (194) or parallel spin transfer from ligand to metal (Table VIII). By contrast, the Co(ii) complexes exhibit shifts that reflect... 

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