An anion is encapsulated in the center of the new cage compound [Ni6(atu)8X]X3 (X=Cl—for the structure see picture—or Br; Hatu=amidinothiourea). A combination of Lewis acid–base and hydrogen-bonding interactions cause the square-planar [Ni(Hatu)2]2+ units, after deprotonation, to assemble to form this compound. A remarkable feature is the anion dependence of the cage formation; nitrate, acetate, and perchlorate are unsuitable as templates.