The long-awaited disclosure of promethium’s enigmatic chemical properties

A new compound containing promethium, one of the world’s rarest elements, has revealed its mysterious properties for the first time.

Promethium exists in nature in very small amounts. The Earth’s crust contains only half a kilogram of this element. In 1945, researchers at Oak Ridge National Laboratory in Tennessee succeeded in producing promethium as a byproduct of the Manhattan Project’s plutonium enrichment program. Due to its nuclear origin, it was named after the Greek mythological giant Prometheus, who stole fire and brought it to humanity.

Promethium is now routinely produced in tiny amounts from the radioactive decay of uranium, and can be incorporated into simple chemical compounds for uses such as glow-in-the-dark paints and nuclear batteries. But its extremely radioactive nature means it is inherently unstable, making it difficult to form long-lasting compounds that are easy to study. Also, the crystal structures in which promethium exists exert forces on promethium’s chemical bonds, obscuring its fundamental chemical properties, such as the length of atomic bonds and how it forms with other compounds.

Now, Alexander Ivanov at Oak Ridge National Laboratory, scientists have found a way to form promethium compounds in water that counter some of the harmful effects of radiation, avoid the obscuring effects of crystal structure, and allow the team to study the element’s chemistry in detail for the first time.

First, they synthesized a compound called bispyrrolidine diglycolamide (PyDGA), based on a molecule that forms compounds with elements similar to promethium. When they added promethium to this molecule in solution, it formed the compound Pm-PyDGA, which has a bright pink color due to its electronic structure.

Ivanov and his team then exposed the compound to X-rays and measured which frequencies were absorbed, revealing how promethium is chemically bonded. This showed that the bond length between promethium and nearby oxygen atoms was about a quarter of a nanometer, which was consistent with the computer simulations they performed.

“This is a very beautiful chemical reaction, and it’s really fun to see the delicate pink color of this complex.” Andrea Serra At University College London.

Information about promethium’s binding behavior could help improve the process of producing purer samples in large quantities from radioactive waste, and could also be used to design new medical compounds for radioactive imaging and cancer therapy. Yes, says Ivanov. “This kind of fundamental information can help drive new technologies,” he says.