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I have a confusion about the correct structure of tetraiodine nonoxide. I came to know about it while reading the equation :

$$\ce{6 I2 (dry) + 11 O3 -> 3I4O9 + 3O2 }$$

Now again I saw the equation :

$$\ce{ I4O9 <=>I^3+(IO3-)3}$$

Now this equation and this is structures in chemistry stack exchange make me to understand that ($\ce{I4O9}$) has the following structure:

enter image description here

enter image description here

But when I saw the following structure (shown below 👇🏻) on Wikipedia I have a doubt about that which structure is correct:

enter image description here

On seeing all the structures, according to me, due to the small size of oxygen to iodine (large size) are unstable on it, therefore the structure 3 is not possible and structure 1 and 2 which are same structure are the correct structures of $\ce{I4O9}$.

Again after thinking I came across the result that the structure on Wikipedia has more chance to be correct because Wikipedia has made this page after my search but the place where I saw the structure 1 and 2 the writer has doubt on itself and this structure is a assumption by him.

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  • $\begingroup$ The 2019 edition of Holleman Wiberg talks about oxidation states, not structure: i.sstatic.net/x08bv.png. Other sources mention a polymeric ionic structure. The sources mentioned by the OP say that their structure suggestions are hypothetical. $\endgroup$
    – Karsten ♦
    Commented Sep 9, 2023 at 17:20

1 Answer 1

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The Wikipedia structure for $\ce{I4O9}$ is plausible, but there is a twist — literally.

The simpler oxide $\ce{I2O5, O2I–O–IO2}$ indicates that a bridging oxygen between two iodine atoms is indeed possible. From Wikipedia:

I2O5 is bent with an I–O–I angle of 139.2°, but the molecule has no mirror plane so its symmetry is C2 rather than C2v. The terminal I–O distances are around 1.80 Å and the bridging I–O distances are around 1.95 Å.[1]

The angle at the central oxygen is much larger than that found in molecules where oxygen is bonded to small atoms or groups (cf. 104.5° in water, 112° in dimethyl ether, from Libretexts). Also the lack of mirror-image symmetry (which makes $\ce{I2O5}$ chiral) implies that the small size of the atom does have a steric effect, forcing the rest of the molecule to twist in exchange for the oxygen serving as a bridge. We may expect the same thing in $\ce{I4O9}$.

Cited reference

  1. Selte, K.; Kjekshus, A. (1970). "Iodine Oxides: Part III. The Crystal Structure of I2O5" (PDF). Acta Chemica Scandinavica. 24 (6): 1912–1924. doi:10.3891/acta.chem.scand.24-1912
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  • $\begingroup$ Two questions. (1) Can you try again? (2) Is there a stylus for Android phones? $\endgroup$
    – Oscar Lanzi
    Commented Sep 10, 2023 at 10:37
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    $\begingroup$ Molecuke?! More like molecute... Anyway, I don't find the bent surprising at all. Iodate is pyramidal in shape. And that is an electronic effect. The twist might well be due to "steric repulsion", but that's also just an electronic effect. Think about biphenyls, even a proton causes a twist. Steric effects are generally explained wrong though. $\endgroup$
    – Martin - マーチン ♦
    Commented Sep 10, 2023 at 10:59
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    $\begingroup$ Determination of the absorption cross-sections of higher order iodine oxides at 355 nm and 532 nm - Scientific Figure on ResearchGate. Available from: researchgate.net/figure/… [accessed 10 Sep, 2023] $\endgroup$
    – Martin - マーチン ♦
    Commented Sep 10, 2023 at 11:20
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    $\begingroup$ Why use Android phone — which is intended for content consumption — for content production and complain about the inconveniences it causes when someone points out typos and lack of proper formatting? Why not just get a laptop and stop torturing yourself (it's evident you are trying your best, but the platform fights you back) while doing tech writing? This problem of yours persist for at least three years, maybe it's the time for a change? $\endgroup$
    – andselisk ♦
    Commented Sep 11, 2023 at 7:47

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