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Xenon_tetrafluoride ($\ce{XeF4}$) is a well-known square-planar molecule with no dipole moment. Molecules are thus have to be bound to each other by London dispersion forces (LDF) which are known to not be very strong (I think even for large molecules they aren't extremely strong).

I was surprised to see that under normal conditions $\ce{XeF4}$ is a crystalline solid. Does this mean that LDF are actually strong enough for $\ce{XeF4}$ to form a solid?

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    $\begingroup$ Quadrupole's gotta be big here. $\endgroup$
    – Mithoron
    Commented May 5 at 21:47
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    $\begingroup$ Great point, but quadrupole is the next order correction after dipole so can it really be that strong? $\endgroup$
    – Mikhail Petrov
    Commented May 5 at 21:54
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    $\begingroup$ At what temperature and pressure?? $\endgroup$
    – DrMoishe Pippik
    Commented May 5 at 21:56
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    $\begingroup$ @DrMoishePippik It's just a solid, but sublimes relatively easily. $\endgroup$
    – Mithoron
    Commented May 5 at 23:01
  • $\begingroup$ Be aware that a zero dipole moment of a molecule does not mean bonds are not polar. It just means that dipole moments of bonds mutually cancel each other. Like in $\ce{CO2}$ case. $\endgroup$
    – Poutnik
    Commented May 6 at 9:00

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The quadrupolar interaction is of some importance here. The structure below, reported by The Materials Project, shows each molecule differently oriented from its neighbors; such a difference in orientation between neighboring molecules makes the quadrupole-quadrupole interaction attractive.

XeF4 crystal structure

The fact that xenon tetrafluoride sublimes unless placed under higher than atmospheric pressure suggests that a liquid phase in which the molecules would not stay in this alignment is significantly less stable than this orientation-ordered solid. As for the strength of this interaction, xenon tetrafluoride is not going to be confused with magnesia or thoria, but it requires a higher temperature than strongly polar, hydrogen-bonded water to reach one atmosphere vapor pressure.

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    $\begingroup$ Solid carbon dioxide has basically the same molecular arrangement, also held together primarily by permanent quadrupole interactions. $\endgroup$
    – TooTea
    Commented May 6 at 15:59
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    $\begingroup$ And carbon dioxide, too, requires elevated pressure to access a liquid phase. $\endgroup$
    – Oscar Lanzi
    Commented May 6 at 16:36

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