Neil Bartlett's superpowerful oxidants NiF6- and AgF4- and the preparation of RhF6.

In recent years I've been interested in the inorganic room temperature molten salt formed by cesium fluoride and hydrogen fluoride, in which liquid is formed with a ratio (at the eutectic point) in a ratio of 2.3 molecules of HF to one molecule of CsF.

Generally fluorine gas, an important industrial agent for a variety of reasons, is formed by electrolysis of a related system in which potassium fluoride KF, complexes similarly with HF. However this system requires significant heat to melt, thus raising the energy cost associated with the preparation of F₂ gas.

This is discussed in a nice paper in the Journal of Fluorine Chemistry published in 2006: Cesium fluorohydrogenate, Cs(FH)2.3F (Journal of Fluorine Chemistry Volume 127, Issue 10, October 2006, Pages 1339-1343)

In the introduction the authors write:

As I was poking around looking for this paper, I came across a paper by Neil Bartlett who is famous among chemists for his discovery, in 1962, that xenon, until then thought to be totally inert, could form compounds. Since 1962 many xenon compounds, most often fluorides but also oxides and other complex compounds, have been discovered, some by Bartlett himself.

The Barlett paper to which I refer, published just two years before his death in 2008 is this one:

Low temperature preparation and uses of potent oxidizers (Journal of Fluorine Chemistry Volume 127, Issue 10, October 2006, Pages 1285-1288)

Normally silver exhibits one oxidation state, 1+, although the 2+ state, analogous to that of its cogener copper, is well known.

Nickel's most common oxidation state is 2+, although a 3+ oxidation state is well known.

The Barlett chemistry is described in the graphic of his reactions found in the paper which may be seen by simply accessing the abstract. In this case, however, Barlett has prepared silver in the 3+ oxidation state, and nickel in the 4+ state, extremely unusual oxidation states.

These compounds, which are fluorine complexes are powerful oxidizing agents, and, as detailed in the paper, they are useful to prepare the hexafluorides of both rhodium and ruthenium (as well as platinum.)

Why is this important?

In less than 15 years the world supply of rhodium from ores, an important industrial catalyst and material with important technological implications, is expected be smaller than the supply available in used nuclear fuels. It is not clear that the element will be available from geological sources at economically recoverable levels at all in the next 20 to 50 years.

At that point it may become necessary to secure rhodium from used nuclear fuels, since that supply from ores will either have been depleted, or insufficient to meet demand for the metal.

Hexafluorides are known for 18 elements, and for all of them, the compounds are either gases or low boiling liquids. Historically used nuclear fuels have been recycled using complexes formed in solvents - the Purex process - which leaves a fair amount of chemical waste and other by products.

A far superior approach will be pyroprocessing including electrolytic recovery, perhaps in molten salts, including, but not limited to inorganic molten salts. (Organic room temperature molten salts - aka "ionic liquids" - are well know and vastly discussed compounds also of potential utility in the recovery of metals from complex mixtures like used nuclear fuels.)

Access to the hexafluorides of ruthenium and rhodium - both elements fairly large constituents of the fission products in used nuclear fuels - will allow for their recovery by simple distillation from the salts, a very clean process compared to extraction. In the case of rhodium in particular, although ruthenium is also an important and valuable element.

I note that uranium, neptunium, and plutonium, which are also valuable constituents of used nuclear fuels also form hexafluorides. The oxidation of americium to its highest state - AmF₅ - also makes its separation from the lanthanide fission products and curium much easier to perform, although, regrettably, not as a gas.

Interesting and important implications I think, exciting chemistry to chemists of a certain type.

Have a nice Sunday afternoon.