Actinides, coordination compounds

Moskvin, a. I. 1975. Mechanism of the interchange of ligands in actinide coordination compounds. Koord. Khim. L8.3-92. 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

The preceding discussion of the relationships between excited state electronic structure and photochemical reactivity focused primarily upon coordination compounds containing cP or low-spin cP transition metals. These relationships are generally applicable, however, to complexes of other d transition elements, the lanthanides and the actinides. A brief survey of the photochemical reactions of these latter systems is presented below. 

In most instances, the magnetic structure of a compound can be understood to be based on interacting localized spin centers, such as classical 3d/4d/5d transition metal ions and 4f lanthanide or 5f actinide cations with unpaired electrons. Note that while the assumption of localized moments is valid for many compounds comprising such spin centers, even partial electron delocalization in mixed-valence coordination compounds renders many localized spin models inapplicable. 

This volume of the Handbook illustrates the rich variety of topics covered by rare earth science. Three chapters are devoted to the description of solid state compounds skutteru-dites (Chapter 211), rare earth-antimony systems (Chapter 212), and rare earth-manganese perovskites (Chapter 214). Two other reviews deal with solid state properties one contribution includes information on existing thermodynamic data of lanthanide trihalides (Chapter 213) while the other one describes optical properties of rare earth compounds under pressure (Chapter 217). Finally, two chapters focus on solution chemistry. The state of the art in unraveling solution structure of lanthanide-containing coordination compounds by paramagnetic nuclear magnetic resonance is outlined in Chapter 215. The potential of time-resolved, laser-induced emission spectroscopy for the analysis of lanthanide and actinide solutions is presented and critically discussed in Chapter 216. 

Among the natural and artificial radioactive elements (Tc, Pm, Po, Fr, Ra, Ac, and actinides), coordination and organometallic compounds of only technetium and the actinide series (An) are well represented at the present time. The interest in their metal complexes has been motivated by the extended use of Tc, available in kilogram amounts, for medical and technical purposes, meanwhile actinides are important on their own for the nuclear industry. A lot of original papers, reviews, and chapters of some books are dedicated to Tc and An complexes [263-281], In the present section, dedicated to the coordination and organometallic chemistry of the actinides and Tc, we intend to present the synthetic techniques for these compounds according to their ligand nature. 

Coordination compounds of diphosphazane dioxides with uranyl or thorium ions were synthesized [475], The crystal structure of [U02(N03)2L1] [L, = Ph2P(0) N(Ph)P(0)Ph2] reveal the bidentate chelating mode of binding of the diphosphazane dioxide to these metals. The chemistry of other uranium organophosphorus and related compounds is described [476-479]. Some of the actinide complexes are presented in Table 5.16. 

The lanthanides, elements 58 through 71, constitute a so-called inner transition series, as do the actinides, elements 90 through 103. Scandium (21) and yttrium (39), together with the lanthanides, are traditionally referred to as the rare earth elements. The lanthanides, with 3+ ions and decreasing radii, show strong ionic bonding and weaker covalent bonding characteristics. As discussed below, the lanthanides tend to exhibit hard sphere or A-type behavior in their coordination compounds. 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

Many of the classic partitioning processes rely on the formation of Am" to facilitate separation from trivalent lanthanides or heavier trivalent actinides. Americium(VI) can be prepared in basic aqueous solutions from Am using powerful oxidants, such as peroxydisulfate, and from Am using weaker oxidants, such as Ce. It can be precipitated from solution as a carbonate by electrolytic or ozone oxidation of concentrated carbonate solutions of Am or Am, or solubilized by dissolution of sodium americyl(VI) acetate. These oxidations and the resulting coordination compounds have been used for relatively large scale processing. For examples, Stephanou et found that Cm could be separated from Am by oxidizing the latter to Am with... 

The crystal chemistry of phosphate minerals has recently been reviewed [9, 10]. These references present a stmctural hierarchy based on the pol5mierization of polyhedra of higher bond-valence, especially tetrahedra and octahedra. In a similar fashion, an extensive stmctural hierarchy of uranyl minerals and inorganic compounds has been developed over the last decade [11, 12]. This chapter follows the concepts and principles of both of these stmctural hierarchies, but places the primary emphasis on actinide coordination. As the coordination environments of the actinides differ with valence state [13, 14], it has been found convenient to discuss the compounds of the lower valence-state actinides separately from those of the higher valence-states. 

Nai+x[Zr2-xRx(P04)3] with Xmax < 1, where R is a rare earth cation (Fig. 10) [97-102], One can suppose that a similar situation can be observed for actinide(III) compounds (or solid solutions). However, none of them has been reported to the present time. An overview on known structures of actinide and similar lanthanide phosphates described above demonstrates a wide variety of coordinations for tri- and tetravalent actinides. The ThOn, UOn, and some LnOn polyhedra with n changing from 10 to 6 are shown in Fig. 12. 

Diketones. Beta-diketones such as acetylacetone, benzoyl-acetone, and isopropyltropolone are well known for their applications in analytical extraction of actinides. These compounds are weak acids due to tautomerization thus they can act as cation exchange extractants. Trivalent actinide [M(III)] extraction by the reagent (HA) at low aqueous acid concentration where the compound behaves both as cation exchanger and coordinator probably follows the reaction... 

Not surprisingly, vibrational spectra have proven to be an invaluable tool for experimental chemists in the characterization of transition metal and actinide sandwich compounds (98). Most known actinocenes have been characterized early on by vibrational spectroscopy (99). The IR and Raman spectra of thorocene and the IR spectra of protactinocene and uranocene were reported in the 1970s (100,101). However, normal coordinate analysis of these vibrational spectra is difficult because of the large number of vibrational modes involved. So far only a tentative assignment of the vibrational spectra of thorocene and uranocene, based on a qualitative group theory analysis, has been advanced (102). 

Formation of these extractable complexes involves coordination bonds with the metal cation, i.e., the sharing of electrons from the complexing agent to complete previously unfilled orbits of the cation. The alkalies and alkaline earths are not easily capable of forming such compounds because they have no empty electron orbits, and hence cannot be readily extracted with organic solvents immiscible with water. On the other hand, elements of the transition groups, such as the rare earths, uranium and the other actinides, iron, nickel, and cobalt, form coordination compounds with ease and are readily extracted by organic solvents immiscible with water. 

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