Actinide complexes Subject

The remainder of this section will focus on true SBMs, which have been the subject of vigorous research. Despite the electron deficiency of early transition metal, lanthanide, and actinide complexes, several groups reported that some of these d f" complexes do react with the H-H bond from dihydrogen and C-H bonds from alkanes, alkenes, arenes, and alkynes in a type of exchange reaction shown in equation 11.32. So many examples of SBM involving early, middle, and late transition metal complexes have appeared in the chemical literature over the past 20 years that chemists now consider this reaction to be another fundamental type of organometallic transformation along with oxidative addition, reductive elimination, and others that we have already discussed. 

With growing interest in the chemical behaviour of actinide ions in the environment (1), the complexation of these ions with carbonate anions has been recently attracting particular attention (2-10) due to the ubiquitous presence of carbonate ions in nature (11, 12) and their pronounced tendency to form complexes with heavy metal ions (7, 10-14). In spite of the carbonate complexation of actinides being considered important chemical reactions for understanding the chemistry of actinides in natural fluids, not many experiments have been devoted up to now to the quantitative study of the subject, though numerous qualitative observations are discussed in the literature. Although there are a few papers reporting the formation constants of carbonate complexes... 

The redox reactions of the actinide elements have been the subject of a recent and authoritative review by Newton and Baker . The net activation process concept is used to interpret the experimental data. Empirical correlations shown to exist include those between the entropies of the activated complexes and their charges, and, for a set of similar reactions, between AG and AG , and and A/f . The present state of the evidence for binuclear species is discussed. 

The application of these methods is described in some detail for recovery of base metals and platinum group metals in Sections 9.17.5-9.17.6 focusing mainly on solution-based hydrometal-lurgical operations, largely those involving solvent extraction, because the nature of the metal complexes formed is usually best understood in such systems. NB. Extraction of lanthanides and actinides is not included as this subject is treated separately in Chapters 3.2 and 3.3. 

The CASSCF/CASPT2 method has been designed to deal with quantum chemical situations, where the electronic structure is complex and not well described, even qualitatively, by single configurational methods. The method relies on the possibility to choose an active space of orbitals that can be used to construct a full Cl wave function that describes the system qualitatively correct. When this is possible, the method is capable of describing complex electronic structures quite accurately. Examples of such situations are found in excited states, in particular photochemical reactions that is the subject of this book, but also in transition metal, and actinide chemistry. 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

The structural chemistry of six-coordinated fluoro complexes of the d-transition elements is the subject of an entire review paper by Babel (7). In contrast, six coordination is rare in actinide fluoro-complexes. The relatively large sizes of the actinide ions (Fig. 3) suggest that low coordination numbers should be found only in the fluoride complexes of the higher oxidation states. The radius ratio, r+/r, predicted for the lower hmit of stabihty of octahedral coordination is ]/2 — 1, which corresponds to a positive ion radius of 0.55 A in coordination with fluoride (1.33 A). 

The synthesis of lanthanide and actinide compounds was the subject of a book (Meyer and Morss 1991). Detailed information is given on the synthesis of lanthanide fluorides (Muller 1991), binary lanthanide halides, RX3 (X=Cl,Br,l) (Meyer 1991a), complex lanthanide(in) chlorides, bromides and iodides (Meyer 1991b), and on two alternative routes to reduced halides, the conproportionation route (Corbett 1991) and the action of alkaU metals on lanthanide(lll) halides (Meyer and Schleid 1991). Therefore, a brief outline of the main preparative routes and synthetic strat es might be sufficent. 

However, our object in this chapter is to cover unstable f-electron systems so that our discussion of the above subjects is brief and, at least in 4f materials, we select one or two examples only. For actinides the situation is more complex, as we shall see below. 

Investigations may be carried out on the tracer level, where solutions are handled in ordinary-sized laboratory equipment, but where the substance studied is present in extremely low concentrations. Concentrations of the radioactive species of the order of 10 m or much less are not unusual in tracer work with radioactive nuclides. A much larger amount of a suitably chosen non-radioactive host or carrier is subjected to chemical manipulation, and the behavior of the radioactive species (as monitored by its radioactivity) is determined relative to the carrier. Thus the solubility of an actinide compound can be judged by whether the radioactive ion is carried by a precipitate formed by the non-radioactive carrier. Interpretation of such studies is made difficult by the formation of radiocolloids, and by adsorption on glass surfaces or precipitates. Tracer studies provide information on the oxidation states of ions and complex-ion formation, and are used in the development of liquid-liquid solvent extraction and chromatographic separation procedures. Tracer techniques are not applicable to solid-state and spectroscopic studies. Despite the difficulties inherent in tracer experiments, these methods continue to be used with the heaviest actinide and transactinide elements, where only a few to a few score atoms may be available [11]. 

There is a great deal more to be said about the actinide ions in solution, about hydrolytic phenomena, and about complex-ion formation. For a comprehensive and authoritative treatment of these important subjects, the reader is referred to Chapter 21 of this work. 

---