# Oxidation state of tungsten in W3O8

Recently I came across this equation:

$$\ce{WO_3 +SnCl_2 +HCl\rightarrow W_3O_8 + H_2SnCl_6 +H_2O}$$

The compound $\ce{W_3O_8}$ that appears in the right hand side of the equation seems to be mixed oxide. Normal calculation of the oxidation state of W in the compound yields $\ce{\frac{16}{3}}$ which is a fraction. A google search on the compound does not give any good results too.

So, is $\ce{W_3O_8}$ a mixed oxide? What are the oxidation states of the different tungsten atoms?

• This might be helpful. researchgate.net/profile/Jun_Li36/publication/…
– JM97
Jan 7 '17 at 15:44
• this dx.doi.org/10.1063/1.3505689 article claims that some 'extra' electrons on W are localized on specific atoms. So it seems that the compound may be classified as mixed oxide. However, it belongs to a weird family of Magneli phases, which are ... somewhat special. They are not non-stechiometric per se, but close to it. I'm not qualified enough to give a full conclusion, though. Jan 14 '17 at 11:58

TL; DR: If you are forced to list the formal oxidation states, then with a high level of confidence it's tungsten(V,VI) octaoxide $$\ce{W^{VI}W^V2O8}$$. However, the real product of this reaction would be a number of non-soichimetric tungsten(IV,V,VI) oxides. Formation of molecular oxoclusters or peroxo compounds is very unlikely.

Reduced tungsten oxides are generally non-stoiciometric. Glemser et al. [1] reduced tungsten(VI) oxide hydrates $$\ce{WO3 * H2O}$$ and $$\ce{WO3 * 2 H2O}$$ with $$\ce{Zn}$$ in $$\ce{HCl}$$ to the blue tungsten oxide hydrates of the general formula $$\ce{WO_{3‐x}(OH)_x}$$ $$(x → 0.5)$$ with the lowest reducing degree of $$2.65$$, which is very close to the one in $$\ce{W3O8}$$ $$(8/3 = 2.6(6))$$.

I would expect that $$\ce{SnCl2}$$ in acidic media would have comparable reducing effect. With the moderately mild reducing agent such as tin(II) chloride there is very little chance of ending up exclusively with $$\ce{W3O8}$$. Also, a presence of a reducing agent also eliminates a possibility of the formation of the peroxo-compounds, so that with a high level of confidence we are dealing with oxide $$\ce{O^2-}$$ ligand.

I find it equally improbable that any cluster formation takes place. For the existence of a polyoxotungstate cluster the terminal $$\ce{W=O}$$ bonds must be "disarmed" in order to prevent further olation/oxolation and aggregation into bulk metal oxide. This is often achieved via the template effect, however even reduced by 1 to 6 electrons forms of 12-heteropolytungstates $$\ce{[(P,Si)W12O40]^3-}$$, the phosphate- or silicate-templated Keggin structure — one of the most stable — only exist briefly in solution [2, pp. 341–343]. In the presented system there is no suitable anion to preserve the three-membered $$\ce{W3O8}$$ ring, and it's very likely going to hydrolyze quickly.

To sum these thoughts up, the more appropriate way to write the reaction would be

$$\ce{n WO3 + SnCl2 + 4 HCl → W_nO_{3n-1} + H2SnCl6 + H2O}$$

where $$\ce{W_nO_{3n-1}}$$ is homologous series analogous to the mixed-valence tungsten blues. On crystallographic level this can be seen as withdrawal of oxygen atoms from $$\ce{WO3}$$, which is isostructural to $$\ce{ReO3}$$ (Fig. 1a). New point defects are eliminated by introducing a crystallographic shear (CS, Fig. 1b). As the reduction proceeds, the octahedra $$\ce{[WO6]}$$ in this structure changing from corner-sharing to edge-sharing motif (Fig. 1c). (Caption and adapted illustration from [3, p. 1086].)

Figure 1. Crystallographic shear in $$\ce{WO3}$$ (a) the idealized structure of $$\ce{WO3}$$, drawn as corner-shared $$\ce{WO6}$$ octahedra, squares in projection (b) {102} CS planes (arrowed), consisting of blocks of four edge-shared octahedra (c) {103} CS planes (arrowed), consisting of blocks of six edge-shared octahedra.

It's impossible to deduce exact oxidation number for such structure with floating stoichiometry, but sure enough an average oxidation number of tungsten can be found trivially from the $$\ce{W_nO_{3n-1}}$$ formula:

$$\text{O.N.}(\ce{W})\cdot n - 2(3n - 1) = 0 \qquad\to\qquad \text{O.N.}(\ce{W}) = 6 - 2/n,$$

e.g. for $$\ce{W3O8}$$ $$\text{O.N.}(\ce{W}) = 6 - 2/3 = 5.3(3)$$. Boundary case $$n = 1$$ dictates the lowest $$\text{O.N.}(\ce{W}) = 4$$, so that $$\ce{W_nO_{3n-1}}$$ phase consists of a mixed-valence oxide with various ratios of $$\ce{W(VI)}:\ce{W(V)}:\ce{W(IV)}$$. Exact ratio is to be determined from XPS/XANES analysis for a given specimen; spectrophotometry would be difficult to implement as tungsten oxides have negligible solubility in most solvents unless some peroxide or $$\ce{HF}$$ is added. Exact location of the reduced tungsten centers can be averaged by finding CS areas with EM/PXRD analysis.

Now, to the $$\ce{W3O8}$$. In fact, it's already been synthesized by Zakharov et al. as follows [4]:

The sample examined was prepared by partial reduction of $$\ce{WO3}$$ with carbon at high temperature in combination with high pressure. The reaction occurred between the graphite container material and a pressed tablet of $$\ce{WO3}$$ in a closed system ($$T = \pu{1773 K}$$, $$P = \pu{50e5 kPa}$$). [...] The product obtained was a mass containing minute dark-red crystallites. The colour suggested that considerable reduction had taken place.

and two phases were characterized with EM/PXRD. Authors also suggest that $$\ce{W3O8}$$ can hardly be obtained under ambient conditions:

The two phases, both of composition $$\ce{W3O8}$$ $$(\ce{WO_{2.667}})$$, do not appear in the binary $$\ce{W-O}$$ system at ambient pressure. $$\ce{W3O8(I)}$$ has a structure of the $$\ce{U3O8}$$ type (Andresen, 1958), which is denser than that of $$\ce{W3O8(II)}$$. The latter has a new type of structure, although some features are shared with other previously known tungsten oxides ({102} CS structures).

Figure 2. Idealized polyhedral representation of $$\ce{W3O8(I)}$$

Figure 3. Idealized polyhedral representation of $$\ce{W3O8(II)}$$

So, there are two possible combinations for the formal $$\text{O.N.}$$s:

1. $$1~\ce{W(VI)} + 2~\ce{W(V)}$$;
2. $$2~\ce{W(VI)} + 1~\ce{W(IV)}$$.

Unfortunately, oxidation states are not mentioned in the article and I haven't found any citeable sources with the determination of oxidation numbers for $$\ce{W3O8}$$, but the XPS studies of uranium octaoxide $$\ce{U3O8}$$ indicated the presence of two oxidation states, $$\ce{U(VI)}$$ and $$\ce{U(V)}$$, in a $$1:2$$ ratio, respectively. Since phase $$\ce{W3O8(I)}$$ is isostructural, there is a high chance that the scenario 1, $$1~\ce{W(VI)} + 2~\ce{W(V)}$$, is quite plausible.

Having the CIFs of both structures at hands, I've been trying to perform bond-valence analysis, but with the common tabulated empirical parameters I haven't reached any meaningful results. Mixed-valenced tungsten and molybdenum oxides often contain delocalized $$\mathrm{d}$$ electrons, making bond-valence analysis alone less meaningful [5].

### References

1. Glemser, O.; Weidelt, J.; Freund, F. Genotypische Oxidhydrate des Wolframs. Zur Frage der Wolframblauverbindungen. Zeitschrift für anorganische und allgemeine Chemie 1964, 332 (5–6), 299–313. https://doi.org/10.1002/zaac.19643320511.
2. Advances in Inorganic Chemistry and Radiochemistry; Emeléus, H. J., Sharpe, A. G., Eds.; Advances in inorganic chemistry and radiochemistry; Academic Press: New York, 1967; Vol. 10. ISBN 978-0-08-057859-0.
3. Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: Chichester, West Sussex, England ; Hoboken, NJ, 2005. ISBN 978-0-470-86078-6.
4. Sundberg, M.; Zakharov, N. D.; Zibrov, I. P.; Barabanenkov, Y. A.; Filonenko, V. P.; Werner, P. Two High-Pressure Tungsten Oxide Structures of $$\ce{W3O8}$$ Stoichiometry Deduced from High-Resolution Electron Microscopy Images. Acta Crystallographica Section B Structural Science 1993, 49 (6), 951–958. https://doi.org/10.1107/S0108768193005701.
5. Domenge`s, B.; McGuire, N. K.; O’Keeffe, M. Bond Lengths and Valences in Tungsten Oxides. Journal of Solid State Chemistry 1985, 56 (1), 94–100. https://doi.org/10.1016/0022-4596(85)90256-7.

Note that $\ce{W3O8}$ is a non-stoichiometric compound and it is not that simple compound you may think. It has a very complex structure as such its oxidation state is very difficult to calculate. I am not going in details about the structure of the compound and I will only mention it a non-stoichiometric compound actually a oxygen-deficient cluster.

Please give a good read to Xin Huang et.al., J. Phys. Chem. A 2006, 110, 85-92, the link that @JM97 has posted. It says that the $\ce{O-O}$ is much longer than normal oxide bond and so it is assumed to be a peroxo bond $\ce{O2^2-}$. Now, peroxide has oxidation state of -1. So, assuming $\ce{W3O8}$ to be a simple compound (no stoichiometric defect or any kind of defect), the oxidation state is:

$$3x + 8(-1) = 0$$ $$x = +{8/3}$$

• There is no way a molecular cluster of such composition is going to be produced by the OP's reaction, it only exists in the gaseous phase and is very unstable, not to mention that it's a reducing media (no peroxides). The non-stoichimetric oxides are the so-called Wadsley and Magneli phases $\ce{M_nO_{3n-2}}$ and $\ce{M_nO_{3n-1}}$, whereas $\ce{W3O8}$ polymorphic phases preserve its stoichiometry all right. Feb 5 '19 at 1:52