Alkalide compounds

Question

I have read much more about metallic hydrides but I am totally confused about "inverse alkali hydrides" or "hydrogen alkalides" while reading refer to this. What are these compounds and how do they form? Is it possible for two elements to form two distinct binary compounds with one containing ions reversely charged with respect to other? I have also heard about "calcium natride" ($\ce{CaNa2}$) and similar compounds. Can someone guide me about them?

Answer 1

Reading modern research papers is probably a great way of learning chemistry, but it is also a great way of confusing oneself quite profoundly, since they typically do not bother to tell you what is common and what is exotic.

Indeed, two elements are pretty unlikely to form two distinct binary compounds with reversed charges. But that's not the case here anyway. The thing you are talking about is not a binary compound, it contains not just the two elements, but also a lot of other stuff, and it is not surprising at all that we can do pretty much whatever we want with these two elements. A simpler example can be thought of. Say, in $\ce{COCl2}$ the oxidation state of $\ce{C}$ is positive and that of $\ce{Cl}$ is negative; in $\ce{(CH3)4N+ClO4-}$ it is the other way around. Finding a pair of true ionic compounds related in the same way presents some challenge, since most elements do not readily form ions of both signs, but it can be done too, as you see.

Look at the picture from that paper: See, it is pretty much like all other alkalides: $\ce{H+}$ is hidden inside the cryptand so that it wouldn't grab that extra electron from $\ce{Na-}$ (which it otherwise would).

In the same manner, one can get natrides of other metals, and probably calcium as well. It is not like you simply mix $\ce{Ca}$ and $\ce{Na}$ (that would produce just an alloy, or maybe nothing at all).

Then there is an ionic salt $\ce{CsAu}$ (without any cryptands or anything), but that's another story.