Americium complexes stability

It is apparent that Am3+ forms a stronger complex than Eu3+ with the same ligand. The main factors that may affect the stability of europium and americium complexes are ionic radii and the availability of the /-electrons. In the present case the /-electron participation of the 5/ orbital of Ams+ is possibly more important than the radius factor. If the radius factor was the more important one, Eu3+ with its smaller ionic size would be expected to form a stronger complex than Am3+. However, it is well-known that the 5/ orbitals are more polarizable than the wellshielded 4/ ones. 

Thiocyanate. — On the basis of /-orbital hybridization Diamond [351] predicted the formation of stronger actinide complexes with thiocyanate ion than for the rare earths. Subls and Chopfin [352] have studied the ion exchange behaviour of many actinide and rare earth thiocyanate complexes and have shown that europium is eluted much sooner than americium from Dowex-1 with ammonium thiocyanate. The stability constants for the formation of MSCN2+ and M(SCN)2 complexes for Nd3+, Eus+, Pu3+, Am3+, Cm3+, and Cf34 have been measured [353] and are tabulated in Table 25. It is apparent from the table that the formation... 

The emphasis of work by French researchers has been the use of soft-donor extractants and complexants to enhance actinide/lanthanide group separation (Vitorge 1985, Musikas et al. 1980, Musikas 1985). The relative stability constants for lanthanide and actinide azide complexes reported by Musikas etal. (1980) suggest that hydrazoic acid could function as a useful reagent for this separation. This is confirmed in a later report for Am/Eu separation (Musikas 1985) in which americium extraction is suppressed by complex formation with azide. The separation factors are not very different from those reported by Sekine (1965) using SCN as the complexant in TBP extraction. However, Choppin and Barber (1989) find that, while the trivalent actinide-azide stability constants are somewhat larger than those of the trivalent lanthanides, the complexation enthalpies calculated from the temperature coefficient of the / s do not support the existence of a covalent bonding contribution. 

Europium and gadolinium show in vivo deposition in the skeleton of rats which is broadly comparable to that of americium and curium. However, the liver deposition of the two actinides appears to be almost double that of the two lanthanides (Durbin 1962). In the blood plasma in vivo (as discussed earlier) europium, gadolinium, americium and curium associate with transferrin to about 20% and the stability constants for their transferrin complexes appear to be similar, table 8. The long-term retention of the lanthanides in bone does not appear to have been studied in similar depth to that of the actinides, but animal studies appear to suggest that lanthanide retention time in bone may be a little shorter than for the actinides. By contrast, liver retention appears to be comparable for actinides and lanthanides, at least in rats and mice (Evans 1990). Although care will be needed in the interpretation of the data, it does... 

Americium(rV) is stabilized against reduction (except by its own radiolysis) in, e g., AmP2Wi706i. Related Cm complexes are chemilmninescent during the reduction to Cm by H2O. Even Cl may perhaps be stabilized by phosphotungstates. 

In the assessment of the refining performance of uranium, systematic data has been reported for the chemical properties of uranium complex in various alkali chlorides such as LiCl-RbCl and LiCl-CsCl mixtures [3-5], Information on the coordination circumstance of solute ions is also important since it should be correlated with stability. The polarizing power of electrolyte cations controls the local structure around neodymium trivalent Nd " " as an example of f-elements and the degree of its distortion from octahedral symmetry is correlated with thermodynamic properties of NdClg " complex in molten alkali chlorides [6]. On the other hand, when F coexists with Cr in melts, it is well-known that the coordination circumstances of solute ions are drastically changed because of the formation of fluoro-complexes [7-9]. A small amount of F stabilizes the higher oxidation states of titanium and induces a negative shift in the standard potentials of the Ti(IV)ITi(ni) and Ti(III)ITi(II) couples [7, 8], The shift in redox potentials sometimes causes specific electrochemical behavior, for example, the addition of F to the LiCl-KCl eutectic leads to the disproportionation of americium Am into Am " and Am metal [9],... 

There is no evidence for any binary oxide of Am higher than Am02 [170,353]. However, ternary oxides are known for Am(iii) through Am(vi) [174,177-183]. Stabilization of high oxidation states in complex oxides is frequently observed [87, 179, 326] excellent examples are the thermally stable Cs2Am04 and Ba3 AmOg. Most complex oxides of americium have been prepared by Keller and Hoekstra and their co-workers. 

Complexation and stability of americium ions in various media, including solids used for waste storage, are of increasing importance because of americium occurrence in nuclear process waste. Solutions to the problems of long-term, safe storage of americium must be found. To answer such questions, an increase in the study of americium chemistry is to be anticipated. 

Americium(iv) is also very prone to disproportionation into the trivalent and pentavalent, or even hexavalent, states (see Chapter 8). Americium(iv) can be stabilized [12] by the fluoride ion, a strongly complexing agent preferring the tetravalent state, as will be discussed further below (Section 21.4.2). 

Stepanov AV (1971) Comparative stability of complexes of yttrium, and some rare-earth and actinide elements with anions of oxalic, citric, ethylenediaminetetraacetic and 1,2-cy-clohexanediaminetetraacetic acids. ZhumNeorg Khim 16 2981 2985 Ohyoshi E, Ohyoshi A (1971) Complexes with polybasic acid americium(III) citrate complexes. J Inorg Nucl Chem 33 4265-4273... 

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