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The dissolution of the ternary material K 6 Rb 6 Si 17  in liquid ammonia yields the solvate compound Rb 4 Si 9  ・ 5NH 3 , which contains fourfold negatively charged nine atom silicon clusters Si 9  4− . Using additionally the [2.2.2] cryptand during the dissolution results in the solvate [K(2.2.2- crypt)]2Si 5  ・ 4NH 3  , in which the Si 5  2−  anion is present in the crystal structure. The Si 5  2−  anion has the shape of a nearly ideal trigonal bipyramid. The starting material K 6 Rb 6 Si 17  contains both Si 4  4−  and Si 9  4−  Zintl anions. In ammoniate crystal structures, Si 9  4−  anions are accessible independently of Si 4  4−  anions, and ammonia of crystallisation plays a major role in the observed crystal symmetry. For the cryptate structures of Si 5  2−  and Ge 5  2−  anions ammonia of crystallisation is obligatory despite the loss of crystal symmetry compared to the crystal structures of the heavier homologues Pb 5  2−  and Sn 5  2− .

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Dissolving Silicides: Syntheses and Crystal Structures of New Ammoniates Containing Si 5  2–  and Si 9  4–  Polyanions and the Role of Ammonia of Crystallisation