A team of chemists from the University of California, Irvine, and Los Alamos National Laboratory has created a new oxidation state, Pu+2, of the transuranic element plutonium. A paper reporting this discovery is published in the Journal of the American Chemical Society.

One of the most fundamental properties of an element’s chemical behavior is its oxidation state chemistry.

An oxidation state relates to the number of electrons that are removed from, or added to, a neutral element (oxidation state of 0) to form cations (positive oxidation states) or anions (negative oxidation states).

The oxidation state of an element has huge influence upon chemical behavior and speciation.

Because of this importance, most accessible oxidation states are generally presumed to have already been identified over the last 100+ years for the elements of the periodic table.

The chemistry of plutonium — a transuranic radioactive chemical element with symbol Pu and atomic number 94 — has been extensively studied at Los Alamos as part of its essential national security mission since the Manhattan Project of the 1940s, revealing its chemistry to be among the most complex of all elements.

Six oxidation states are known and have been verified: 0 (metallic form) and +3, +4, +5, +6, +7 in molecular systems.

The current work demonstrates that a seventh formal oxidation state (+2) has now been accessed and verified, representing an unexpected new chemical form of plutonium.

“Over 70 years of chemical investigations have shown that plutonium exhibits some of the most complicated chemistry in the periodic table,” the authors said.

“Six Pu oxidation states have been unambiguously confirmed (0 and +3 to +7), and four different oxidation states can exist simultaneously in solution.”

“We report a new formal oxidation state for plutonium, namely Pu2+ in [K(2.2.2-cryptand)][PuIICp’’ 3 ], Cp’’ = C 5 H 3 (SiMe 3 ) 2 .”

“This finding marks out plutonium, already known for its extremely complex chemistry, as the actinide element with the largest number of confirmed oxidation states,” said lead co-author Dr. Andrew Gaunt, a researcher in the Chemistry Division of Los Alamos National Laboratory.

The research was built on prior work that showed the +2 oxidation state of lanthanides, uranium and thorium could be generated using organometallic anions — cyclopentadienyl rings — to facilitate reduction of molecules containing a +3 metal cation to molecules containing a +2 metal cation.

These +2 molecules were shown to be accessible on account of the ‘organometallic framework’ around the metal ion, which allows the extra electron upon reduction of +3 to +2 to populate a ‘d’ rather than an ‘f’ orbital, which is unusual.

The researchers spent a year at Los Alamos working in a specially equipped radiological laboratory to develop and apply a similar methodology with the much more radioactive isotope of Pu-239.

“Seeing the color change from blue to deep purple upon reduction of a plutonium +3 molecule, and knowing that if the experiment was a success then I would be the first person in the world to see a new formal oxidation state for an element with my own eyes, was quite riveting,” Dr. Gaunt said.

_____