Tetravalent actinide derivatives

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Ureas. Urea adducts (and those of the closely related A-alkylated derivatives) may be prepared from nonaqueous solvents alternatively, preparation in aqueous alcoholic solution leads to the formation of hydrates. In contrast to the carbamides discussed above, there is relatively little variability in the coordination number of reported urea adducts of tetravalent actinides. Most complexes are either six- or seven-coordinate higher coordination numbers are observed for the larger thorium ion (Table 15). 

The most promising actinide squestering agents yet prepared (Table IX) are the sulfonated catechoylamide derivatives of linear tetra-amines. These compounds appear to strongly bind tetravalent actinides, while only weak complexation has been observed for trivalent and divalent metals. A derivative of the natural product spermine, 3,4,3-LICAMS, is more effective in plutonium removal at low dosages than any other sequestering agent tested to date. 

Uranocene and its ring alkylated, 3-7 arylateds-io and annulatedn-i3 derivatives constitute the majority of the [8]annulene complexes known for the tetravalent actinides. The synthesis of the other bis([8]annulene)actinide(IV) complexes is similar to that of the uranium systems. 

In Table 20.7 are listed radii of trivalent actinide ions (coordination number CN 6) derived from measurements on trichlorides by the method of Bums, Peterson, and Baybarz [288]. Determinations of M-Cl distances have been made for M = U, Pu, Am, Cm, and Cf the other values were estimated by use of unitcell data and curve fitting. All these radii are relative to the trivalent lanthanide radii of Templeton and Dauben [396], who employed data from cubic sesquioxides and assumed atomic positions to establish M-O distances. Also included in Table 20.7 are radii of tetravalent actinide ions obtained from the M-O distances calculated from unit-cell parameters of the dioxides [1] by subtracting 1.38 A for oxygen (the value used [396] for the sesquioxides). For comparison. Shannon s ionic radii, derived from oxides and fluorides, and Peterson s tetravalent radii, derived from dioxides, are shown [537,538]. As... 

The absence of reliable thermodynamic data for the tetrafluorides has contributed to difficulties in defining the chemistry of the rare earth elements. The fact that only Ce, Pr, and Tb form stable Rp4(s) phases has been established (see section 2.4) however, the thermochemistry of these fluorides has remained uncertain. Insight is provided by the work of Johansson (1978), who has correlated data for lanthanide and actinide oxides and halides and derived energy differences between the trivalent and tetravalent metal ions. The results, which have been used to estimate enthalpies of disproportionation of RF4 phases, agree with preparative observations and the stability order Prp4< TbP4 < CeP4. However, the results also indicate that tetravalent Nd and Dy have sufficient stability to occur in mixed metal systems like those described by Hoppe (1981). 

The calix[4]arene molecule was modified at both the upper (para) and lower (phenolic) rims by the introduction of phosphate and phosphonate groups, mainly by the group of Kalchenko (Fig. 4B). Such derivatives complex and allow the extraction of lanthanide and actinide tri- and tetravalent cations. Detoxification of radioactive waters may be possible in this way, as the chemieal and physical robustness of the calix[n]arenes makes them particularly suitable for this task. 

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