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The biggest crown ether experimentally confirmed is 81-crown-27 1 How can this molecule form complexes when the distance between the 27 oxygen atoms which complexes with the cation inside seems extremely large, making coordination seems extremely hard.

Moreover, from my understanding of coordination bonding, it seems that there is an extreme excess of electron sharing from the aforementioned oxygen atoms, to the point that it seems electrostatic repulsion should take over. The discovery is only observational in nature, so this in not addressed. I might be making wrong assumptions here, so clarify if needed. Thank you.

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    $\begingroup$ I guess you're imagining a huge circle with tiny cation inside? Well, I'm imagining rather something like a braid with cation coordinatin' to a little loop $\endgroup$
    – Mithoron
    Commented Jul 24, 2022 at 18:07
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    $\begingroup$ Most crown ethers are very flexible, not rigid, and can reorganise themselves to achieve decent coordination with, in the case of 81-crown-27, probably many ions. $\endgroup$
    – matt_black
    Commented Jul 24, 2022 at 18:21
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    $\begingroup$ For such enormous macrocyclic polyethers, coordination with metal cations is probably very closely emulated by simply dissolving metal salts in polyethylene glycol solutions (which is in fact alluded to in the abstract of the article). $\endgroup$
    – Nicolau Saker Neto
    Commented Jul 24, 2022 at 22:18

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The article does not directly describe a complex, only the neat ether, so there is no indication of how a complex may be formed. Most likely, an alkali metal cation would be complexed with only some of the oxygen atoms, the rest being in loops along the ligand between the metal-ligand linkages. For example, six oxygen atoms could be connecteded to the metal with three or four others between adjacent metal-ligand linkages.

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