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.

Plutonium
The periodic table entry for plutonium (Pu) showing +2 added as a new entry to the list of known oxidation states with a ball-and-stick/molecular orbital depiction of the Pu2+-containing molecule. 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)][PuIICp”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 5f66d0 with a low-lying (and possibly thermally accessible) 5f56d1 state. The results demonstrate that plutonium is likely a crossover element in the actinide series at which point a pure 5fn+1 configuration becomes more stable for the +2 cations than configurations containing 6d character.

Source : U.S. Department of Energy