As reported in Nature Chemistry, a praseodymium compound is proposed with an formal oxidation state of just +5 (A.C. Boggiano et al. DOI). Praseodymium (Pr) is one of those rare-earth metals that countries trade wars over. In nature the element is trivalent as a phosphate or a carbonate. In the laboratory an example is praseodymium(III) fluoride. The +3 oxidation state is the preferred one because with a Pr electron configuration of [Xe]4f³6s², 3 electrons can be removed fairly easily but the the remaining two are tightly bound because the f-orbital (halter-shaped) in which they reside can penetrate the electron cloud right to the atomic nucleus.

That said, the only Pr5+ compound discovered thus far is praseodymium(V) oxide nitride but it only lives a few degrees above absolute zero. The new Pr5+ compound with full name tetra([(tri(tert-butyl)phosphanylidene)amino])praseodymium Hexafluorophosphate does much better and withstands temperatures up to -30 degrees and also does not mind X-ray diffraction. One thing that helps is a full body-armour of 36 methyl groups sticking out.

In 2024, the lead investigators Boggiano and La Pierre of the Georgia Institute of Technology had already reported the synthesis of tetravalent tetra([(tri(tert-butyl)phosphanylidene)amino])praseodymium (DOI) by reaction of the THF complex of praseodymium triiodide with tri(tert-butyl)phosphanylidenamine and benzylpotassium followed by an oxidation reaction with silver iodide. In the new work this compound is simply oxidized further, now with silver hexafluorophosphate at -60 degrees in THF. The advantage of this oxidizing agent, the article explains, was that the silver(0) particles formed could be easily removed by filtration. A solution is stable up to -35 degrees due to thermal degradation at higher temperatures. According to the article the counter ion is responsible for this degradation but the supporting information only gives a thermal degradation mechanism of a related compound but with a ligand group degrading under the influence of the infamous proton intruder.

And the evidence for the proposed structure? You might expect it but no synthesis was published. On the other hand, the oxidation reaction was predicted with cyclic voltammography, crystalline flakes were photographed, the x-ray structure looked the part and NMR showed the all the right couplings. And what is next? Remember that for praseodynium going beyond +5 is pointless because that would require messing with the near-perfect closed-shell xenon electron configuration. But one further down the lanthanide line we have neodymium and a quick literature search learns this element is still stuck at +4 with a potential for +6!


CRD data: here