Would you still call neutral and positively-charged molecular metal oxides polyoxometalates, or do you know a better/proper way to name this class of compounds (Polyoxometalloids/Polyoxometallic species/Polyoxometallides/...)?

This question occurred on the premise of an interesting paragraph in the paper (1) regarding the nomenclature of newly-synthesized polynuclear oxovanadium-lanthanide species:

Given that polyoxovanadates are increasingly being used as diamagnetic ‘‘ligands’’ for lanthanide ions, the cage structure in 3 could be described as a tubular $\ce{[V^V12O32]^4-}$ ligand coordinating to two $\ce{\{Dy(OH2)4\}^3+}$ centres, with the latter plugging the ends of the tube and trapping the interstitial chloride. However, it is surely preferable in this case to consider the dysprosium units as part of the cage, and this to be a heteropolyoxovanadium complex. One must also note that, very unusually, 3 is a cationic cage system, and that ‘‘heteropolyoxovanadate’’, with the -ate suffix denoting a negative charge is here incorrect according to normal IUPAC nomenclature rules. On the other hand, there are examples of positively-charged oxygen-bridged metal cages which correspond to well known motifs from polyoxometalate chemistry such as the $\ce{\{Al^{III}13\}}$ $\varepsilon$-Keggin-ion structure $\ce{[AlO4Al12(OH)24(H2O)12]^7+}$.

Compound $\ce{[AlO4Al12(OH)24(H2O)12]^7+}$ (2) was investigated in 1960, though it has not been explicitly denoted as polyoxoaluminate at that time.

Here are the structures of these polynuclear complexes:

enter image description here

It also seems like IUPAC does not really care, always referring to polyoxometalate term. So please feel free to share your opinion!

(1) Šimuneková, M.; Prodius, D.; Mereacre, V.; Schwendt, P.; Turta, C.; Bettinelli, M.; Speghini, A.; Lan, Y.; Anson, C. E.; Powell, A. K. RSC Advances 2013, 3 (18), 6299. DOI 10.1039/c3ra40385h

(2) Johansson, G.; Gullman, L.-O.; Kjekshus, A.; Söderquist, R. Acta Chemica Scandinavica 1960, 14, 771–773. DOI 10.3891/acta.chem.scand.14-0771


Many years ago ago I dealt with [PMo$_{12}$O$_{40}$(VO)$_2$]$^{q−}$, which is basically a Keggin polyoxomolybdate (-3) capped with two vanadyl groups (2*(+2)). It was in a highly reduced form, but depending on how many blue electrons were residing in the Keggin cage, it might conceivably present neutral or positive charge. My work was theoretical and actually revolved around the control of this reduction state.

While I did not find any canonical source to support it, when privately thinking about the possibility of a positively charged polyoxometalate, I always called the q>0 version of the molecule a polyoxometalonium. I did this in analogy with the well-known positively charged polyatomic ions ammonium, hydronium... but also other similar species, I guess, such as sulfonium, phosphonium. [Folk linguistics warning: no IUPAC was involved in this nomenclature].

Back then I did not think about the neutral (q=0) version, but I'd say polyoxometallic complex sounds a reasonable approximation to me. To me it does not inherently say "no charge", but it sure does not imply negative (nor positive) charge. To make the meaning clear, I'd go for neutral polyoxometallic complex.


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