Cyclopentadienyls actinides

Gas Phase Photoelectron Spectra of d- and /-Block Organometallic Compounds Table 10. Ionization energies (eV) of lanthanide and actinide cyclopentadienyl metal halides... 

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

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... 

Table 14.11 Some representative actinide cyclopentadienyl complexes.

As already noted, the carbonylation of bis(pentamethyl-cyclopentadienyl) actinide hydrocarbyls is irreversible in the cases studied thus far. Thus, thermolysis does not result in CO loss, but rather in interesting chemical reactions. Thermolysis of 1 ( 5) in toluene solution results in hydrogen atom migration to yield an enolate (eq.(5)). NMR studies establish that eq.(5) is essentially quantitative, and that the stereochemical course... 

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. 

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,... 

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. 

Pentalenediyl compounds, cyclopentadienyl actinide complexes, 4, 221 Pentalenes... 

The cyclopentadienyl reactions have been illustrated for early as well as late transition metals and also the actinides. It is remarkable that the reactions are very similar across the entire group. This methodology results often in sandwich-type complexes of the Et units and has been found successful for both phosphorus and arsenic, but only one example has been reported for antimony. There are no reports of bismuth metal reacting with cyclopentadienyl metal fragments. Neither Sb nor Bi readily form the soluble E4 species as do P and As, and so their reactivities may be limited by solubility factors (Table XLI). 

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