Complexes of the Lanthanides and Actinides

Related complexes are obtained with Pd, where one face of a benzene ring is shared by two Pd atoms . Mono-f/ -benzene ligands on a pentacobalt cluster also form from the aluminum halide-aluminum metal conditions . The preparation of -arene complexes of the lanthanides and actinides has been more diflScult, presumably because of the proclivity of these metals to form ionic bonds. Reactions between hexamethylbenzene and metal halides in the presence of AlClj-Al or AlCl-Zn reagents in the isolation of a mononuclear Sm complex, in addition to di- and trinuclear U complexes. ... 

Recently the first examples of nitrosyl complexes of the lanthanides and actinides have been reported by Evans and co-workers, and they are illustrated in... 

Most textbooks discuss transition metal complexes separately from those of the main group elements. There is, in fact, much in common between the two classes and, whenever possible, we shall treat them as one. However, complexes of the transition metal ions may possess an incomplete shell of d electrons which necessitate separate discussion. This characteristic makes it particularly useful to determine the magnetic and spectral properties of members of this class of complexes and the exploration of these properties will require separate chapters devoted to them. In a similar way, complexes of the lanthanide and actinide elements, with, typically, an incomplete shell of f electrons tucked rather well inside the atom and away from the ligands—and so behaving rather as if they are in an isolated atom—require their own discussion. 

Had the f electrons in complexes of the lanthanides and actinides experienced crystal fields comparable to those in complexes containing d electrons, we would next have to include these fields, just as was done in Chapters 7 and 8. But these fields are small—much more important... 

In order to understand the photochemical reactions of metal complexes at the molecular level, it is necessary to know both the number and the energy levels of the spectroscopic states of the complex. The first step in developing a state model is to know the coordination number and structure of the complex about the metal center. For complexes of the lanthanide and actinide ions the coordination number is commonly 8 or 9, but for transition metal complexes a coordination number of 6 is that most frequently observed. 

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

The primary reason for the thermal stability or instability of the alkyls and aryls of the lanthanides and actinides is kinetic in nature. As in all kinetic processes, the rate of reaction is dependent upon the activation energy between reactants and products. By considering the various decomposition pathways and factors which enhance or inhibit these pathways one can rationalize the observed sta-bihties of the various complexes. 

Although several phenyl derivatives of the lanthanides and actinides have been characterized, only one re-arene complex of the / transition metals is known to date. This is the uranium(III) benzene complex, U(AlCl4)s CeHe 153), prepared by the combination of uranium tetrachloride, aluminum trichloride and aluminum powder in refluxing benzene, the Fischer-Hafner method [154). The molecular geometry of the complex is shown in Fig. 18. 

Fig. 23. The spanning of the g-edges by hexafluoroacetylacetonates in the tetrakis-complexes of the lanthanides and the actinides...

FIGURE 1.29 The seven /-orbitals of a shell (with n > 3) have a very complex appearance. Their detailed form will not be used again in this text. However, their existence is important for understanding the periodic table, the presence of the lanthanides and actinides, and the properties of the later d-block elements. A darker color denotes a positive lobe, a lighter color a negative lobe. 

The solution reactions of Cm3+ closely resemble those of the lanthanide and actinide +3 ions, and the fluoride, oxalate, phosphate, iodate, and hydroxide are insoluble. Complexes appear to be weaker than those of preceding elements. 

With almost all of the conceivable coordination chemistry of the expanded porphyrins still left to be explored, it cannot be over-stres that the potential for new chemistry is enormous. This is i rticularly true when account is made of the fact that the chemistry of the metalloporphyrins has played a dominant role in modern inorganic chemistry. What with the possibility to enhance the stability of imusual coordination geometries (and, perhaps oxidations states) and the ability to form stable coordination complexes with a variety of unusual cations including those of the lanthanide and actinide series, the potential for new inorganic and organometallic discoveries are almost unlimited. For instance, as with the porphyrins, one may envision linear arrays of stacked expanded porphyrin macrocycles which may have unique conducting properties and/or which could display beneficial super- or semiconducting capabilities. Here, of course, the ability to coordinate not only to cations but also to anions could prove to be of tremendous utility. 

A more extensive set of actinide complexes is formed with tungstates of the Keggin and Dawson structure, An[XWi 1039)2 and AnpC2Wi706i]2 (X = P, Si, B, As An = Th, U, Np, jj ggg ligands form very stable complexes of tetravalent lanthanides and actinides. 

Although the elements of the lanthanide and actinide series have long been known to exhibit a quite extensive organometallic chemistry, it is only within the last decade that typical sandwich species have been prepared and studied. These systems however, although resembling the familiar metallocene and bis-arene compounds of the d-block elements, are not strictly their analogues since in both f-orbital series the known sandwich complexes are derived only from the cyclooctatetraenyl dianion. 

The second series of inner transition elements, the actinides, have atomic numbers ranging from 90 (thorium, Th) to 103 (lawrencium, Lr). All of the actinides are radioactive, and none beyond uranium (92) occur in nature. Like the transition elements, the chemistry of the lanthanides and actinides is unpredictable because of their complex atomic structures. What could be happening at the subatomic level to explain the properties of the inner transition elements In Chapter 7, you ll study an expanded theory of the atom to answer this question. 

The known structures of the lanthanide and actinide metals are indicated in table 5.01, from which it will be seen that the structures characteristic of the true metals, and particularly the hexagonal close-packed arrangement, are common. Polymorphism, however, is of frequent occurrence among these elements, and plutonium, for example, crystallizes in no fewer than six modifications—the A1 and A2 structures indicated and four others of greater complexity. Praseodymium, neodymium and samarium are of interest in that they possess close-packed structures in which the sequence of layers is... 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

No complexes of scandium or of yttrium are known. The complexes of the lanthanides and of the actinides will be considered in Sections I and J. 

NUG/BAY] Nugent, L. J., Baybanz, R. D., Burnett, J. L., Ryan, J. L., Electron-transfer and f-d absorption bands of some lanthanide and actinide complexes and the standard (II-III) oxidation potential for each member of the lanthanide and actinide series, J. Phys. Chem., 77, (1973), 1528-1539. Cited on page 646. 

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