Actinides complexes, stability

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. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

Proposed species in granitic groundwater are given in Table VIII. Since many of the relevant stability constants are not known, especially for actinide complexes with OH", CO32" and PO ", any final conclusive predictions of speciation in groundwater are impossible. 

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... 

As typical hard acids, the stabilities of the actinide complexes are due to favorable entropy effects. The enthalpy terms are either endothermic or very weakly exothermic and are of little importance in determining the overall position of the equilibrium in complex formation. 

Figure 15.18 Comparison of complexation stability constants for the interaction of various ligands with different actinide oxidation states (Kim, 1986).

This brief outline serves to illustrate the ways in which the stoichiometries of extraction reactions may be determined experimentally. A full account of extraction equilibria and their use in measuring stability constants for actinide complexation, as well as reaction stoichiometries, is given by Ahrland el al.9a... 

P-Diketonates. Very strong actinide complexes with p-diketones [An(acac)4 and An02(acac)2] are used in solvent extraction and separation of actinides. They are prepared by direct interaction of the metal or actinyl halide with the appropriate p-diketone in the presence of a base. Only fluorinated An(IV) diketonates produce adducts with Lewis bases, whereas common An02(acac)2 (An = Np, Pu) are stabilized by adduct formation. Fluorinated U02(hfa)2 is a very strong Lewis acid and its adducts with H20 and ROH can be sublimed without decomposition [282],... 

Also electrons of other inner orbitals than d may of course give rise to covalent bonding, provided they are on suitable energy levels. It is thus very likely that the abnormal stability of certain actinide complexes is due to partially covalent bonds involving 5/electrons (5). On the other hand there are no signs that the 4/electrons of the lanthanides are able to participate in such bonds evidently these are situated too deep within the atom. 

Aside from the neutral tris(amido)actinide complexes that have been prepared with sterically encumbering ligands as described, an alternate approach to the stabilization of trivalent actinide amides is the generation of anionic ate -type complexes. As an example, reaction of Ul3(THF)4 with excess KHNAr (Ar = 2,6-Pr 2CgH3) produces the anionic complex [K(THF)2]2[U(NHAr)5] which has been crystallographically characterized. ... 

Hydroxamate. Hydroxamate complexes of trivalent actinides can be prepared directly in aqueous solution and other polar solvents and extracted into organic solvents, but due to the high thermodynamic stability of the corresponding tetravalent actinide complexes they are rapidly oxidized. They can also be prepared in solution via electrochemical reduction of the tetravalent complexes. These complexes have been studied for their role in separating high and low valent actinides in nuclear fuel processing schemes. ... 

Actinide complexes, 1129-1220 coordination numbers, 1131 geometry, 1131 kinetic stability, 1130 steric effects, 1130 Aciinide(III) complexes, 1131-1136 amides, 1133 ammines, 1131 antipyrine, 1134 aqua,1133... 

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