A New Oxidation State for Plutonium

Plutonium has more verified and accessible oxidation states than any other actinide element, an important insight for energy and security applications.

Image courtesy of Josh Smith, Los Alamos National Laboratory

The Science

Associated with nuclear power and national security, plutonium (Pu) has some of the most complicated chemistry on the periodic table. For the first time, scientists showed a new oxidation state (electron arrangement) for element 94. They achieved this result by adding electrons to a specific plutonium-containing molecule, an organometallic Pu³⁺-containing complex. They then studied the results. The results included a complex containing Pu²⁺. The Pu²⁺ was bound to carbon in the complex. This is the first example of plutonium (Pu²⁺) bonding to carbon in the literature.

The Impact

An oxidation state of an element is one of the most influential properties. It dictates chemical behavior, bonding, and reactivity. Because of this importance, scientists have heavily studied the range of accessible oxidation states for all the elements. They generally presumed that all possibilities were already defined. Discovery of a new oxidation state represents a significant breakthrough in the fundamental chemistry of plutonium. This work could lead to development of new bonding types and reactivity patterns. The results offer insights for security and energy production.

Summary

The researchers synthesized a Pu(III) starting material, PuCp”₃ (Cp” = the substituted anionic cyclopentadienyl ring [C₅H₃(SiMe₃)₂-1,3-]–), and characterized it by single crystal X-ray diffraction to reveal the first structural information on Pu–C bonds. Subsequent reduction with potassium graphite (in the presence of 2.2.2.-cryptand to encapsulate the K+ ion) affords [K(cryptand)][Pu^IICp”₃], which formally contains a Pu²⁺ ion, never before isolated and verified in molecular form. The compound is highly sensitive to oxygen and water (which very rapidly re-oxidize the plutonium back to the +3 state). In the analogous La²⁺, Th²⁺ and U²⁺ systems, stabilization of the 5d or 6d orbitals by the C₃ symmetric arrangement of the Cp” ligands affords fⁿd¹ (La, U) or d² (Th) electron configurations. In contrast, computational analyses of the Pu²⁺ molecule, in conjunction with ultraviolet/visible/near infrared spectroscopic data, indicate an electronic picture best described as predominately 5^f66d⁰ with a low-lying (and possibly thermally accessible) 5^f56^d1 state. The results demonstrate that plutonium is likely a crossover element in the actinide series at which point a pure 5^fn+1 configuration becomes more stable for the +2 cations than configurations containing 6d character.

Contact

Andrew J. Gaunt, Stosh A. Kozimor
Los Alamos National Laboratory
[email protected], [email protected]

William J. Evans
University of California, Irvine
[email protected]

Funding

U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program (experimental work); DOE Graduate Student Research Program (student support); National Science Foundation (theoretical component).

Publications

C.J. Windorff, G.P. Chen, J.N. Cross, W.J. Evans, F. Furche, A.J. Gaunt, M.T. Janicke, S.A. Kozimor, and B.L. Scott, “Identification of the formal +2 oxidation state of plutonium: Synthesis and characterization of {[Pu^II[C₅H₃(SiMe₃)₂]₃}–.” Journal of the American Chemical Society 139, 3970-3973 (2017). [DOI: 10.1021/jacs.7b00706]