Ionic states of embedded atoms in metals

Most atoms, when embedded in a low-density jellium model of a metal, are expected to form negative ions, whereas of course metallic atoms release their valency electrons when embedded in their own free-electron gas. The reason is that there is a critical electron density above which the negative ionic state becomes unstable. It is also unstable when the embedding is into a low-density crystal, instead of jellium, because such a crystal lies on the insulating side of the Mott-Hubbard transition and so cannot supply the embedded neutral atom with an extra electron.