Actinides complexes

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

Bond distance (in pm) and vibrational frequencies (in cm ) of UFe obtained by various density functional methods quasi-relativistic frozen core Becke-Lee-Yang-Parr (QR-FC BYLP), Hay-Martin large core ECP hybrid DFT (HM ECP B3LYP and VWN), Stoll-Preuss small core ECP LDA (SP ECP VWN) and an all-electron scalar relativistic LDA (AE SR VWN). 

In the case of actinyls, where no experimental information on unperturbed gas phase species is available, the assessments were mainly based on a comparison with other computational results, first of all, with the HF-based correlation methods, such as ACPF or CCSD(T), either at the all-electron or relativistic 

Spin-orbit effects were found negligible for U(VI) as well as Np(VI) actinyls and hexafluorides [209]. For U(VI), this result is in accord with the closed-shell electronic structure for Np(VI), it can be rationalized with the localized atomic character of the 5f orbital, occupied by the unpaired electron. 

Uranocene itself was prepared by allowing cyclooctatetraene (COT) to react with potassium in dry, oxygen-free tetrahydrofuran (THF) at —30° followed by the addition of a THF solution of anhydrous uranium tetrachloride  

Following the characterization of U(COT)2, the analogous thorium complex was synthesized 12). Its physical and chemical properties were so different from those of the uranium compound that initially there was question as to whether the complex had the same -sandwich structure. The X-ray structure anal5 is however showed it to be isostructural with U(COT)2 11). Subsequently, Pu(COT)2 13), Np(COT) 13), and Pa(COT)2 14, 15) have been prepared and their X-ray powder patterns show them all to be isostructural with U(COT)2. 

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

It is interesting to compare these actinide(IV) cyclooctatetraene complexes with similar compounds of the group IVB transition elements Ti, Zr and Hf. Bis (cyclooctatetraene) complexes of aU three are known although structural data is only available for the first two. All would appear to involve both planar and non-planar COT rings and to exhibit a sHpped sandwich structure rather than the true sandwich structure of uranocene. 

A wide range of rotomeric configurations has previously been observed for the substituted metallocene series of tc-CbHs organometaUic complexes of the transi- 

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The phosphido complex, Th(PPP)4 [143329-04-0], where PPP = P(CH2CH2P(CH2)2)2) has been prepared and fully characterized (35) and represents the first actinide complex containing exclusively metal—phosphoms bonds. The x-ray stmctural analysis indicated 3-3-electron donor phosphides and 1-1-electron phosphide, suggesting that the complex is formally 22-electron. Similar to the amido system, this phosphido compound is also reactive toward insertion reactions, especially with CO, which undergoes a double insertion (35,36). 

Actinide complexes with carboxylic acids. U. Casellato, P. A. Vigato and M. Vidali, Coord. Chem. Rev., 1978, 26, 85-159 (276). 

Idem, Part 12 - Fuger, J. Khodakovskij, I.L. Medvedev, V.A. Navratil, J.D. "The Actinide Complex Ions with Inorganic Ligands", in preparation. 

Little is known about actinide complexation by humic or fulvic acids although logg -values for Ara3+, Thlt+ and U022+ with humic acid at pH 4.0-4.5 as 6.8, 11.0 and 5.8, respectively, are reported (43). 

Anions of the type [M(C2B9H11)2] (M = Fe, Cp, Ni) were also used as noncoordinating anions with [Cp2ZrMe] +, which are active for the polymerization and copolymerization of ethylene and oc-olefins in non-polar solvents such as toluene and hexane [54]. By using the same anions, cationic actinide complexes have also been prepared [110]. 

Nuclear Magnetic Relaxation Studies on Actinide Ions and Models of Actinide Complexes Jean F Desreux... 

Hydrogenation with Early Transition Metal, Lanthanide and Actinide Complexes... 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Hydrogenation Catalysts Based on Group III, Lanthanide, and Actinide Complexes I 131... 

Table 6.17 Hydrogenation of alkenes catalyzed by actinide complexes.

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