Actinide complexes thiocyanates

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... 

Acoustic emission diazine metal complexes, 80 Acrylonitrile metal complexes, 263 Actinide complexes cupferron, 510 dimethyl sulfoxide IR spectra, 490 phosphines SHAB theory, 1040 phthalocyanines, 864 thiocyanates, 236 Actins, 973 Acylates H3-ligands... 

Sato, T., Kotani, S., Good, M. L., Extraction of certain anionic actinide complexes from aqneous solntions by long chain alkyl ammo-ninm compounds - 11 Thorium(lV)-thiocyanate system, J. Inorg. Nucl. Chem., 35, (1973), 2547-2554. Cited on pages 373, 374, 544, 565. 

Table 25. Comparison of the stability constant data of the Eus+ thiocyanate complex with that of Nd3+ and some actinides (p = 1.0 M at 25° C)...

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

As the anhydrous actinide thiocyanates are not known (only hydrated [M(NCS)4(H20)4], thiocyanate complexes are prepared metathetically. 

The Am thiocyanates have been studied intensively because of the separation of lanthanide and actinide elements in thiocyanate media. Three complexes of general formula Am(SCN) n = 1-3) have been identified from spectroscopic and solvent extraction data. 

For the thiocyanate complexes of lanthanides or actinides, the coordination occurs via the nitrogen atom (electronegativity 3.0). ... 

Another possibility is to apply the double-double effect to the differentiation of outer-from inner-sphere complexes of lanthanides and actinides because both the magnitude and direction of the effect depend on the difference in the electron-donor ability between water in the aquoion and the ligand in the complex. Therefore, a substantial difference between outer- and inner-sphere complexes with respect to the double-double effect should exist. Research in this field, using lanthanide complexes with thiocyanates as an example, is being made in our laboratory. 

Studies of the complex-ion chemistry of Es have been made in conjunction with measurements of the stability constants of other trivalent actinides. A summary of the known stability constants for einsteinium complexes is shown in Table 12.5. With the possible exception of the two lower thiocyanate complexes, chloride is the only outer-sphere complex in which water of hydration lies between the ligand and einsteinium ioa The remaining complexes are believed to be inner-sphere as inferred from the increase of a given stability constant with increase in atomic number and from the enthalpy and entropy of formation of the complex. 

The thiocyanate complexes are all weaker than the corresponding carboxylate or sulfate complexes. The pattern found for the affinity to actinide ions of different charge types is very similar to that found for the ligands discussed... 

The formation of thiocyanate complexes of actinide(iii) ions (Section 21.4.6) has also been successfully utilized for the separation of actinides from lanthanides by means of anion exchange [188,201]. From a 5 m ammonium thiocyanate solution, americium (iii) is strongly sorbed on an anion-exchange column while the lanthanides pass through [201]. In this way, americium (iii) can be almost completely separated from lanthanides present even in very large amounts. 

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