Chemistry:Hexachloroplutonate

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The hexachloroplutonate ion, PuCl2−
6, is an ion containing plutonium and chlorine. It forms several salts, which are called hexachloroplutonates.[1][2]: 1104 

Structure and bonding

The PuCl2−
6 ion features a slightly distorted octahedral molecular geometry, with three distinct Pu-Cl bonds. The Pu-Cl bond lengths vary between 2.574 and 2.598 Å, and the Cl-Pu-Cl bond angles range between 88.54° and 91.46°.[3]

The Pu-Cl bonds in PuCl2−
6 are primarily ionic; however, they feature small but important amounts of covalent bonding. The chloride ligands bond using their s and p electrons, while plutonium bonds using its s, d, and f electrons. The Pu-Cl bond order is less than 1, and the shape of the Pu-Cl bond slightly deviates from cylindrical, possibly indicating a small amount of pi bonding.[3] The 5f electrons in the PuCl2−
6 have a large nephelauxetic effect[4] (decrease in electron-electron repulsion).

In aqueous solution

It is the dominant species of plutonium(IV) in 11 M hydrochloric acid solution.[5]

Salts

The PuCl2−
6 ion forms several salts. Dicaesium hexachloroplutonate (Cs
2PuCl
6) is a pale yellow solid which is precipitated from concentrated HCl solution by the addition of caesium chloride. Cs
2PuCl
6 can be used to prepare other plutonium compounds, such as the cyclopentadienide complex (η5−C
5H
5)
3PuCl, and it has been proposed for use in the purification of plutonium metal. It has been used as a reference for plutonium(IV), and due to its stability, it has been proposed for use as a primary standard for plutonium.[2]: 839, 1104, 1191 [5] Potassium and rubidium hexachloroplutonates, K
2PuCl
6 and Rb
2PuCl
6, are also known. They are prepared by heating mixtures of plutonium(III) chloride and the respective alkali metal chlorides under high chlorine pressures.[6]

PuCl2−
6 is also known to form salts with quaternary ammonium cations, including tetramethylammonium, tetraethylammonium,[2]: 1104  and tetrabutylammonium.[7] The tetramethylammonium and tetraethylammonium salts, [NMe
4]
2[PuCl
6] and [NEt
4]
2[PuCl
6], form orange-yellow crystals.[2]: 1109  [NEt
4]
2[PuCl
6] was used in the first synthesis of plutonocene.[8] The tetrabutylammonium salt, [N(n-Bu)
4]
2[PuCl
6], is prepared by the addition of tetrabutylammonium chloride to plutonium(IV) in HCl aqueous solution. It is soluble in several organic solvents.[7]

PuCl2−
6 is also known to form salts with pyridinium, pyridinium dervatives,[3][9] and tetraphenylphosphonium.[10]

References

  1. ↑ Wilson, Richard E. (2015). "Structure, Phase Transitions, and Isotope Effects in [(CH3)4N]2PuCl6". Inorganic Chemistry 54 (21): 10208–10213. doi:10.1021/acs.inorgchem.5b01288. PMID 26225472. https://doi.org/10.1021/acs.inorgchem.5b01288. 
  2. ↑ 2.0 2.1 2.2 2.3 Clark, David L.; Hecker, Siegfried S.; Jarvinen, Gordon D.; Neu, Mary P. (2011). "Plutonium". The Chemistry of the Actinide and Transactinide Elements. doi:10.1007/978-94-007-0211-0_7. ISBN 978-94-007-0211-0. https://www.xylenepower.com/THE_CHEMISTRY_OF_THE_ACTINIDE_AND_TRANSA.pdf. 
  3. ↑ 3.0 3.1 3.2 Surbella, Robert G.; Ducati, Lucas C.; Schofield, Mark H.; McNamara, Bruce K.; Pellegrini, Kristi L.; Corbey, Jordan F.; Schwantes, Jon M.; Autschbach, Jochen et al. (2022). "Plutonium Hybrid Materials: A Platform to Explore Assembly and Metal–Ligand Bonding". Inorganic Chemistry 61 (45): 17963–17971. doi:10.1021/acs.inorgchem.2c02084. PMID 36305869. https://www.osti.gov/servlets/purl/1957814.  (Supporting information: https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.2c02084/suppl_file/ic2c02084_si_001.pdf)
  4. ↑ Ryan, J.L.; Jørgensen, Chr. Klixbüll (1964). "Electron transfer and 5f→6f transitions in uranium(IV), neptunium(IV), plutonium(IV) hexahalides". Molecular Physics 7 (1): 17–29. doi:10.1080/00268976300100781. Bibcode: 1964MolPh...7...17R. https://doi.org/10.1080/00268976300100781. 
  5. ↑ 5.0 5.1 Wang, Yufei; Rice, Natalie T.; Knapp, Julia G.; Adelman, Sara L.; Aldrich, Kelly E.; Arko, Brian T.; Besmer, Manuel L.; Gilhula, J. Connor et al. (2025). "Insights for controlling plutonium behavior in hydrochloric acid solutions". Inorganic Chemistry Frontiers 12 (14): 4392–4408. doi:10.1039/D5QI00409H. https://doi.org/10.1039/D5QI00409H. 
  6. ↑ Morss, Lester R.; Fujino, Takeo (1988). "Preparation and crystal structures of plutonium(IV) hexachloro complexes, Rb2PuCl6 and K2PuCl6". Journal of Solid State Chemistry 72 (2): 338–352. doi:10.1016/0022-4596(88)90038-2. https://doi.org/10.1016/0022-4596(88)90038-2. 
  7. ↑ 7.0 7.1 Reilly, Sean D.; Scott, Brian L.; Gaunt, Andrew J. (2012). "[N(n-Bu)4]2[Pu(NO3)6] and [N(n-Bu)4]2[PuCl6]: Starting Materials to Facilitate Nonaqueous Plutonium(IV) Chemistry". Inorganic Chemistry 51 (17): 9165–9167. doi:10.1021/ic301518g. PMID 22900742. https://doi.org/10.1021/ic301518g. 
  8. ↑ Karraker, David G.; Stone, John Austin; Jones, Erwin Rudolph; Edelstein, Norman (1970-08-01). "Bis(cyclooctatetraenyl)neptunium(IV) and bis(cyclooctatetraenyl)plutonium(IV)". Journal of the American Chemical Society 92 (16): 4841–4845. doi:10.1021/ja00719a014. ISSN 0002-7863. Bibcode: 1970JAChS..92.4841K. https://doi.org/10.1021/ja00719a014. 
  9. ↑ Wilson, Richard E.; Schnaars, David D.; Andrews, Michael B.; Cahill, C. L. (2014). "Supramolecular Interactions in PuO2Cl42– and PuCl62– Complexes with Protonated Pyridines: Synthesis, Crystal Structures, and Raman Spectroscopy". Inorganic Chemistry 53 (1): 383–392. doi:10.1021/ic4023294. PMID 24328217. https://doi.org/10.1021/ic4023294. 
  10. ↑ Minasian, Stefan G.; Boland, Kevin S.; Feller, Russell K.; Gaunt, Andrew J.; Kozimor, Stosh A.; May, Iain; Reilly, Sean D.; Scott, Brian L. et al. (2012). "Synthesis and Structure of (Ph4P)2MCL6(M = Ti, Zr, Hf, Th, U, Np, Pu)". Inorganic Chemistry 51 (10): 5728–5736. doi:10.1021/ic300179d. PMID 22554112. https://doi.org/10.1021/ic300179d. 



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