Cyclopentadienyls lanthanides actinides

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

This is remarkable, since the reduction potential of Th(IV) to Th(III) recently has been estimated as —3.7 volts 73) and direct reduction of U(C5H5)4 and Pu(C5Hs)3 with potassium metal produces the actinide metals. The ei/z for naphthalene in acetonitrile is —2.63 V (nearly the same as the aLkaJi metals). Since this is much smaller than the Th(IV) to Th(III) reduction potential, it would seem to imply substantial stabilization of the +3 state by cyclopentadienide. The observed room temperature magnetic moment of Th(C 5115)3 (0.403 BM) is consistent with the Th(III) (5/ ) assignment. Thorium triscyclopentaxhenide is similar in behavior to U(C5H5)3, forms adducts with both THF and cyclohexyhso-nitrile and has been shown to be isostructural with the other tris (cyclopentadienyl) actinides and lanthanides. 

From the relative stabilities of the actinide homoalkyls or -allyls and the tris(cyclopentadienyl) actinide alkyls, it appears that a coordinatively saturated metal center is necessary for kinetic stability. In contrast to f-transition metal alkyls, the absence of hydrogens appears to be of minor importance. In the case of the lanthanide alkyls and the tetrabenzylthorium, where the formal coordination number is only four, the steric bulkiness of the Hgands must be responsible for their observed thermal stability. 

Table 7. Ionization energies (eV) of some cyclopentadienyl and cyclooctatetraenyl lanthanide and actinide compounds...

Fig. 13a-c. Photoelectron spectra of lanthanide and actinide cyclopentadienyl halides a Gdtp-CsHcGCl b U(t,-CsHs)3C1 c U(r -C5Me5)2Cl2... 

This is a special volume of Inorganic Syntheses that focuses on complexes that are likely to be useful as starting materials for the preparations of new transition metal coordination and organometallic compounds. There are chapters on complexes with weakly coordinated and therefore easily displaced ligands, low-valent complexes that undergo oxidative-addition reactions, substituted metal carbonyl complexes, nucleophilic metal carbonyl anions, transition metal clusters, a variety of cyclopentadienyl complexes, lanthanide and actinide complexes, and a range of other useful ligands and complexes. 

With a few exceptions,most tris(cyclopentadienyl) complexes contain lanthanides (Ln) or actinides as the central metal. CpjLn have polymeric structures, with chains of (ri -CpljLn units linked by cyclopentadienides that are bound in an T) -fashion toward one or both of the metals they bridge. Cp Ln and Cp U are monomeric. The Sm(III) complex has been prepared from Sm(II) and Cp jPb (Equation 3.83) the U(III) complex was prepared from the U(III) hydride and tetramethylfulvene (Equation 3.84). ... 

Only the larger atoms of the early 2 and 3 row transition metals, lanthanides and actinides can coordinate three cyclopentadienyl ligands simultaneously. Com-... 

Although the bonding in actinide cyclopentadienyls is more covalent than in the corresponding lanthanide complexes, the halide ligand is labile and can be substituted by a large variety of anionic ligands. The synthetic routes most commonly involve metathetical halide substitution (eq. 8), or protolysis of the U-Cp bond (eq. 9). The properties of most of these derivatives have been summarized by... 

The second review is due to Pepper and Bursten (1991). This review focussed on the electronic structure of actinide-containing molecules. Note that the present chapter complements this in that our chapter is mostly on lanthanide-containing species. Consequently, the reader is referred to the excellent review by Pepper and Bursten (1991) for a comprehensive summary of the electronic structure of actinide-containing species. The review by Pepper and Bursten (1991) contains the details of calculations on actinide hydrides, actinide halides, actinide oxides, cyclopentadienyl-actinide complexes, aetinocenes, metal-metal bonding in actinide systems and miscellaneous other actinide systems. This review also consists of descriptions of theoretical techniques employed to study the actinide-containing molecules. The reader is directed to this review for further details on such calculations on actinide-containing molecules. 

The lanthanides and actinides also form a wide range of complexes with unsaturated organic ligands including allyls, cyclopentadienyls and cycloocta-tetraene derivatives. The bonding is strongly polar. Coordination numbers are determined largely by steric requirements around the metal centre. 

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