This excellent answer describes the appearance of solvated electrons in ammonia as first a blue, and then metallic color. It is produced by dissolving sodium in ammonia at low temperature. According to Wikipedia this is called an electride:

An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of $\ce{[Na(NH3)6]+}$ and solvated electrons:

$$\ce{Na + 6 NH3 → [Na(NH3)6]+e−}$$

The cation $\ce{[Na(NH3)6]+}$ is an octahedral coordination complex.

Question: How to interpret the shape in this representation of an electride?

Included in the Wikipedia article is the image shown below. Is it fairly self-evident to chemists what this image represents? If so could someone help me understand it? I see little things which might represent the location of solvated electrons, and a larger continuous/contiguous? surface which might represent all of the ions and molecules and the fact that any moment most of them are in some degree of bonding with each other, but that's just my guess. What defines the surface, some sort of threshold on electron density?

In this case the original source might be hard to track down, so an "educated guess" by someone familliar with this kind of representation would be welcome.

I'd like to read more about this kind of 3D representation of an ionic solution, so an introductory source would be appreciated.

below: "Cavities and channels in an electride", from here.

enter image description here

  • 1
    $\begingroup$ Yes, it is fairly self-evident: this is a 3D isosurface of God knows what, maybe electron density. Little things look like artifacts. $\endgroup$ Jul 26, 2017 at 7:38
  • $\begingroup$ @IvanNeretin since this particular solution can have such a high solvated electron density that it actually behaves optically as a metal, I'm thinking that in this case the solvated electrons would have to show up as isolated, localized regions of high electron density themselves. But they do look a bit too scrappy for that. $\endgroup$
    – uhoh
    Jul 26, 2017 at 7:52
  • $\begingroup$ If the solvated electrons were isolated, then the whole thing would behave like an insulator and not like a metal in any way. $\endgroup$ Jul 26, 2017 at 7:56
  • $\begingroup$ @IvanNeretin I only said ".. show up as isolated.." in an electron density plot. It's a matter of the arbitrarily chosen density threshold chosen for a given plot. I'm not sure what isolated means in your statement, but I'm just talking about the appearance of the surfaces shown in the plot, nothing more profound than that. I now have more papers to read about solvated electrons, so I will be giving it more thought and study to understand what it really means, but I believe these are truly if transiently unbound electrons, free to respond to external electric fields at optical frequency, no? $\endgroup$
    – uhoh
    Jul 26, 2017 at 8:15
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    $\begingroup$ True, it is a matter of arbitrary threshold, and it can be that the electrons are not really isolated. $\endgroup$ Jul 26, 2017 at 8:24


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