Actinide acts as electron donor for first time

A new thorium-aluminum complex is the first in which an actinide element donates electrons when bonding with a metal (Chem. Sci. 2018. DOI: 10.1039/c8sc01260a).

The complex, synthesized by John Arnold of the University of California, Berkeley, and colleagues, is already unusual because its thorium atom is in a +3 oxidation state. Chemists have made fewer than 10 Th(III) complexes to date. The energy and symmetry of its highest orbitals mean the element is almost always in a +4 oxidation state. Arnold’s group was trying to better understand Th(III)’s electronic structure when they stumbled on some unusual bonding behavior.

To stabilize this reduced form of thorium, the chemists coordinated it to a bulky alanate ligand. Density functional theory (DFT) calculations predicted delocalization of the electron in thorium’s highest occupied orbital onto the aluminum atom. Electronic paramagnetic resonance spectroscopy showed the two atoms were sharing electrons in a covalent interaction. Further DFT studies indicated that thorium was indeed donating an electron to the aluminum.

Stosh Kozimor, an actinide chemist at Los Alamos National Laboratory, thinks this compound could be the first of many like it. “I predict that Arnold and coworkers have uncovered a bonding interaction that may have substantial implications on reactivity that transcends hundreds of compounds containing [actinides] linked to main group Lewis acids,” he says.

Arnold agrees. If chemists want to make more kinds of metal-actinide bonds, it helps if the actinide can be an electron donor and not just an acceptor, he says.

Arnold says the research, which was funded by the U.S. Department of Energy, could be most useful in making alloys of the heavier actinides, like plutonium. In nuclear weapons, these elements are stabilized by metals like gallium. Making and understanding stable alloys is vital for nuclear technology, he says.

CORRECTION: This story was updated on May 3, 2018, to correct the conclusions of the density functional theory calculations of the complex. The calculations indicated that thorium donated one electron to aluminum, not multiple electrons.