Chemistry:Americium compounds

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Americium compounds are compounds containing the element americium (Am). These compounds can form in the +2, +3, and +4, although the +3 oxidation state is the most common. The +5, +6 and +7 oxidation states have also been reported.

Oxides

Three americium oxides are known, with the oxidation states +2 (AmO), +3 (Am
2O
3), and +4 (AmO
2). Americium(II) oxide was prepared in minute amounts and has not been characterized in detail.[1] Americium(III) oxide is a red-brown solid with a melting point of 2205 °C.[2] Americium(IV) oxide is the main form of solid americium which is used in nearly all its applications. Like most other actinide dioxides, it is a black solid with a cubic (fluorite) crystal structure.[3]

The oxalate of americium(III), vacuum dried at room temperature, has the chemical formula Am
2(C
2O
4)
3 · 7H2O. Upon heating in vacuum, it loses water at 240 °C and starts decomposing into AmO
2 at 300 °C, the decomposition completes at about 470 °C.[4] The initial oxalate dissolves in nitric acid with the maximum solubility of 0.25 g/L.[5]

Halides

Halides of americium are known for the oxidation states +2, +3, and +4,[6] where the +3 is most stable, especially in solutions.[7]

Oxidation state F Cl Br I
+4 Americium(IV) fluoride
AmF
4
pale pink
+3 Americium(III) fluoride
AmF
3
pink 		Americium(III) chloride
AmCl
3
pink 		Americium(III) bromide
AmBr
3
light yellow 		Americium(III) iodide
AmI
3
light yellow
+2 Americium(II) chloride
AmCl
2
black 		Americium(II) bromide
AmBr
2
black 		Americium(II) iodide
AmI
2
black

Reduction of Am(III) compounds with sodium amalgam yields Am(II) salts – the black halides AmCl
2, AmBr
2, and AmI
2. They are very sensitive to oxygen and oxidize in water, releasing hydrogen and converting back to the Am(III) state. Specific lattice constants are:

  • Orthorhombic AmCl
    2    : a = 896.3±0.8 pm, b = 757.3±0.8 pm and c = 453.2±0.6 pm
  • Tetragonal AmBr
    2    : a = 1159.2±0.4 pm and c = 712.1±0.3 pm.[8] They can also be prepared by reacting metallic americium with an appropriate mercury halide HgX
    2    , where X = Cl, Br, or I:[9]
Am + HgX
2 (mercury halide) → AmX
2 + Hg (at 400–500 °C)

Americium(III) fluoride (AmF
3) is poorly soluble and precipitates upon reaction of Am+
3 and fluoride ions in weak acidic solutions:

Am+
3 + 3F−
 → AmF
3↓

The tetravalent americium(IV) fluoride (AmF
4) is obtained by reacting solid americium(III) fluoride with molecular fluorine:[10][11]

2AmF
3 + F
2 → 2AmF
4

Another known form of solid tetravalent americium fluoride is KAmF
5.[10][12] Tetravalent americium has also been observed in the aqueous phase. For this purpose, black Am(OH)
4 was dissolved in 15-M NH
4F with the americium concentration of 0.01 M. The resulting reddish solution had a characteristic optical absorption spectrum which is similar to that of AmF
4 but differed from other oxidation states of americium. Heating the Am(IV) solution to 90 °C did not result in its disproportionation or reduction, however a slow reduction was observed to Am(III) and assigned to self-irradiation of americium by alpha particles.[13]

Most americium(III) halides form hexagonal crystals with slight variation of the color and exact structure between the halogens. So, chloride (AmCl
3) is reddish and has a structure isotypic to uranium(III) chloride (space group P63/m) and the melting point of 715 °C.[6] The fluoride is isotypic to LaF
3 (space group P63/mmc) and the iodide to BiI
3 (space group R3). The bromide is an exception with the orthorhombic PuBr
3-type structure and space group Cmcm.[7] Crystals of americium chloride hexahydrate (AmCl
3 · 6H2O) can be prepared by dissolving americium dioxide in hydrochloric acid and evaporating the liquid. Those crystals are hygroscopic and have yellow-reddish color and a monoclinic crystal structure.[14]

Oxyhalides of americium in the form AmVI
O
2X
2, AmV
O
2X, AmIV
OX
2, and AmIII
OX can be obtained by reacting the corresponding americium halide with oxygen or Sb
2O
3, and AmOCl can also be produced by vapor phase hydrolysis:[9]

AmCl
3 + H
2O → AmOCl + 2HCl

Other inorganic compounds

Hydroxide

The only known hydroxide of americium is Am(OH)
3, which is the first compound of americium, discovered in 1944 as part of the Manhattan project. Americium hydroxide is a pink solid[15] which is sparingly soluble in water.[16]

Due to self-irradiation, the crystal structure of 241
Am(OH)
3 decomposes within 4 to 6 months (241
Am has a half-life of 432.2 years); for 244
Cm(OH)
3 the same process takes less than a day (244
Cm has a half-life of 18.11 years).[15]

When ozone is bubbled through a slurry of americium(III) hydroxide in 0.03 M potassium bicarbonate at 92 °C, hexagonal KAmO
2CO
3 (potassium dioxoamericium(V) carbonate) can be obtained. Potassium carbonate can also be used. The resulting KAmO
2CO
3 reacts with dilute acids to produce americium dioxide:[17]

O
3 + Am(OH)
3 + KHCO
3 + H
2O → KAmO
2CO
3 + 3H
2O + O
2

Chalcogenides and pnictides

The known chalcogenides of americium include the sulfide AmS
2,[18] selenides AmSe
2 and Am
3Se
4,[18][19] and tellurides Am
2Te
3 and AmTe
2.[20] The pnictides of americium (243Am) of the AmX type are known for the elements phosphorus, arsenic,[21] antimony, and bismuth. They crystallize in the rock-salt lattice.[19]

Silicides and borides

Americium monosilicide (AmSi) and "disilicide" (nominally AmSi
x, with 1.87 < x < 2.0) were obtained by reduction of americium(III) fluoride with elementary silicon in vacuum at 1050 °C (AmSi) and 1150−1200 °C (AmSi
x). AmSi is a black solid isomorphic with LaSi, it has an orthorhombic crystal symmetry. AmSi
x has a bright silvery lustre and a tetragonal crystal lattice (space group I41/amd), it is isomorphic with PuSi
2 and ThSi
2.[22] Borides of americium include AmB
4 and AmB
6. The tetraboride can be obtained by heating an oxide or halide of americium with magnesium diboride in vacuum or inert atmosphere.[23][24]

Organoamericium compounds

Predicted structure of amerocene, (η8−C
8H
8)
2Am

Analogous to uranocene, americium forms the organometallic compound amerocene with two cyclooctatetraene ligands, with the chemical formula (η8−C
8H
8)
2Am,[25] but it is still hypothetical up to date. An anionic complex KAm(COT)2 can be prepared by reacting K2COT and AmI3 in THF.[26] A cyclopentadienyl complex is also known that is likely to be stoichiometrically AmCp
3.[27][28]

Formation of the complexes of the type Am(n–C
3H
7–BTP)
3, where BTP stands for 2,6-di(1,2,4-triazin-3-yl)pyridine, in solutions containing n–C
3H
7–BTP−
 and Am+
3 ions has been confirmed by EXAFS. Some of these BTP-type complexes selectively interact with americium and therefore are useful in its selective separation from lanthanides and another actinides.[29]

See also

References

  1. ↑ Akimoto, Y. (1967). "A note on AmN and AmO". Journal of Inorganic and Nuclear Chemistry 29 (10): 2650–2652. doi:10.1016/0022-1902(67)80191-X. 
  2. ↑ Wiberg, p. 1972
  3. ↑ Greenwood, p. 1267
  4. ↑ Penneman, p. 4
  5. ↑ Penneman, p. 5
  6. ↑ 6.0 6.1 Wiberg, p. 1969
  7. ↑ 7.0 7.1 Asprey, L. B.; Keenan, T. K.; Kruse, F. H. (1965). "Crystal Structures of the Trifluorides, Trichlorides, Tribromides, and Triiodides of Americium and Curium". Inorganic Chemistry 4 (7): 985. doi:10.1021/ic50029a013. https://digital.library.unt.edu/ark:/67531/metadc1035960/. 
  8. ↑ Baybarz, R. D. (1973). "The preparation and crystal structures of americium dichloride and dibromide". Journal of Inorganic and Nuclear Chemistry 35 (2): 483. doi:10.1016/0022-1902(73)80560-3. 
  9. ↑ 9.0 9.1 Greenwood, p. 1272
  10. ↑ 10.0 10.1 Asprey, L. B. (1954). "New Compounds of Quadrivalent Americium, AmF4, KAmF5". Journal of the American Chemical Society 76 (7): 2019. doi:10.1021/ja01636a094. 
  11. ↑ Greenwood, p. 1271
  12. ↑ Penneman, p. 6
  13. ↑ Asprey, L. B.; Penneman, R. A. (1961). "First Observation of Aqueous Tetravalent Americium1". Journal of the American Chemical Society 83 (9): 2200. doi:10.1021/ja01470a040. 
  14. ↑ Burns, John H.; Peterson, Joseph Richard (1971). "Crystal structures of americium trichloride hexahydrate and berkelium trichloride hexahydrate". Inorganic Chemistry 10: 147. doi:10.1021/ic50095a029. 
  15. ↑ 15.0 15.1 Krivovichev, Sergey; Burns, Peter; Tananaev, Ivan (2006). Structural Chemistry of Inorganic Actinide Compounds. Elsevier. p. 67. ISBN 978-0-08-046791-7. https://books.google.com/books?id=mV-phntexBQC&pg=PA67. 
  16. ↑ Runde, Wolfgang (2011) (in en), Americium and Curium: Radionuclides, American Cancer Society, doi:10.1002/9781119951438.eibc0438, ISBN 978-1-119-95143-8, https://onlinelibrary.wiley.com/doi/abs/10.1002/9781119951438.eibc0438, retrieved 2020-03-21 
  17. ↑ Template:Cite tech report
  18. ↑ 18.0 18.1 Damien, D.; Jove, J. (1971). "Americium disulfide and diselenide". Inorganic and Nuclear Chemistry Letters 7 (7): 685. doi:10.1016/0020-1650(71)80055-7. 
  19. ↑ 19.0 19.1 Roddy, J. (1974). "Americium metallides: AmAs, AmSb, AmBi, Am3Se4, and AmSe2". Journal of Inorganic and Nuclear Chemistry 36 (11): 2531. doi:10.1016/0022-1902(74)80466-5. 
  20. ↑ Damien, D. (1972). "Americium tritelluride and ditelluride". Inorganic and Nuclear Chemistry Letters 8 (5): 501. doi:10.1016/0020-1650(72)80262-9. 
  21. ↑ Charvillat, J.; Damien, D. (1973). "Americium monoarsenide". Inorganic and Nuclear Chemistry Letters 9 (5): 559. doi:10.1016/0020-1650(73)80191-6. 
  22. ↑ Weigel, F.; Wittmann, F.; Marquart, R. (1977). "Americium monosilicide and "disilicide"". Journal of the Less Common Metals 56: 47. doi:10.1016/0022-5088(77)90217-X. 
  23. ↑ Lupinetti, A. J. et al. U.S. Patent 6,830,738 "Low-temperature synthesis of actinide tetraborides by solid-state metathesis reactions", Filed 4 Apr 2002, Issued 14 December 2004
  24. ↑ Eick, Harry A.; Mulford, R. N. R. (1969). "Americium and neptunium borides". Journal of Inorganic and Nuclear Chemistry 31 (2): 371. doi:10.1016/0022-1902(69)80480-X. 
  25. ↑ Elschenbroich, Christoph (2008). Organometallchemie. Vieweg+teubner Verlag. p. 589. ISBN 978-3-8351-0167-8. 
  26. ↑ Mueller, Werner; Lindner, Roland. Potassium bis(cyclooctatetraenyl)americium(III). Transplutonium 1975, Proc. Int. Transplutionium Elem. Symp., 4th [Conference]. 1976. pp 131-137.
  27. ↑ Albrecht-Schmitt, Thomas E. (2008). Organometallic and Coordination Chemistry of the Actinides. Springer. p. 8. ISBN 978-3-540-77836-3. https://books.google.com/books?id=rgmnVSzFzXMC&pg=PA8. 
  28. ↑ Dutkiewicz, Michał S.; Apostolidis, Christos; Walter, Olaf; Arnold, Polly L. (30 January 2017). "Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding". Chemical Science 2017 (8): 2553–61. doi:10.1039/C7SC00034K. PMID 28553487. 
  29. ↑ Girnt, Denise; Roesky, Peter W.; Geist, Andreas; Ruff, Christian M.; Panak, Petra J.; Denecke, Melissa A. (2010). "6-(3,5-Dimethyl-1H-pyrazol-1-yl)-2,2'-bipyridine as Ligand for Actinide(III)/Lanthanide(III) Separation". Inorganic Chemistry 49 (20): 9627–35. doi:10.1021/ic101309j. PMID 20849125. https://www.escholar.manchester.ac.uk/api/datastream?publicationPid=uk-ac-man-scw:209191&datastreamId=POST-PEER-REVIEW-PUBLISHERS.PDF. 



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