# What is the largest, noncrystalline, non-polymer inorganic molecule?

A question came up in class from one of my chemistry students. He asked what the largest, non-crystal, non carbon-based, molecule is. We ruled out complex proteins, DNA, and other polymers, as well as crystalline structures and metals from the list. I didn't have an answer for him, so I looked it up but could not find a definite answer. I found two threads on Reddit, listed below. Polyoxometalates come up, but without any specifics. I know "largest" can be subjective, so please specify if the molecule is "largest" because of its physical size or because of its mass.

• I noticed someone down-voted my question. Can someone tell me why? – James Mar 5 '20 at 17:17
• You didn't exclude amorphous substances... Another thing is what you call a polymer? Polyoxometalates already have quite repetitive structure. If you link together some big fullerenes, at which point it would become "a polymer"? – Mithoron Mar 29 '20 at 0:24

I agree with the redditors that the answer is likely a polyoxometalate.

$$\ce{Na48[H_xMo368O1032(H2O)240(SO4)48] · \text{ca.}~1000 H2O}$$ aka $$\ce{\{Mo368\}}$$ (core: $$M \approx\pu{60.7 kDa},$$ dimensions $$\approx \pu{2.5 nm} × \pu{4.0 nm})$$ is the primary candidate [1]:

Figure 2. Structure of 1a in crystals of 1 in polyhedral (a) and ball and stick (b) representation perpendicular to the $$C_4$$ axis and along the $$C_4$$ axis ((c) and (d); building units $$\ce{\{Mo1\}}$$ yellow, $$\ce{\{Mo2\}}$$ red, $$\ce{\{Mo(Mo5)\}}$$ blue with blue-turquoise pentagonal bipyramids; $$\ce{O}$$ atoms small red spheres, $$\ce{S}$$ atoms gray spheres). Note that one capping part is turned against the other by $$360/16^\circ.$$ The different building blocks and the arrays of different local symmetry are assigned.

A recent review of mass-spectrometry studies of large polyoxometalates is also available [2].

### References

1. Müller, A.; Beckmann, E.; Bögge, H.; Schmidtmann, M.; Dress, A. Inorganic Chemistry Goes Protein Size: A $$\ce{Mo368}$$ Nano-Hedgehog Initiating Nanochemistry by Symmetry Breaking. Angewandte Chemie International Edition 2002, 41 (7), 1162–1167. DOI: 10.1002/1521-3773(20020402)41:7<1162::AID-ANIE1162>3.0.CO;2-8.
2. Surman, A. J.; Robbins, P. J.; Ujma, J.; Zheng, Q.; Barran, Perdita. E.; Cronin, L. Sizing and Discovery of Nanosized Polyoxometalate Clusters by Mass Spectrometry. J. Am. Chem. Soc. 2016, 138 (11), 3824–3830. DOI: 10.1021/jacs.6b00070.
• Are these things really molecules? – Alchimista Mar 6 '20 at 8:20
• @Alchimista. Yes, they are. I'm not sure about this one, but majority of polyoxometalates is also stable in appropriate solution, and some even in gas phase. – andselisk Mar 6 '20 at 10:30
• Isn't the prefix "poly" a give away vs the question? – Beerhunter May 23 '20 at 13:37
• @Beerhunter "Poly-" in polyoxometalates (POMs) usually refers to the presence of more than three metal centers linked via oxo-bridges. Under given conditions, one usually doesn't get all sorts of molecules/chains/blobs etc. of POMs, only one type of species. There is no molecular mass distribution to speak of, as one would, for example, in case of traditional organic polymers. – andselisk May 23 '20 at 15:21
• @andselisk so the nomenclature states that if its three or more, tri, tetra etc aren't used and the reference is simply "poly"? – Beerhunter May 23 '20 at 18:15