Dissolving Silicides: Syntheses and Crystal Structures of New Ammoniates Containing Si5 and Si9 Polyanions and the Role of Ammonia of Crystallisation

The dissolution of the ternary material K6Rb6Si17 in liquid ammonia yields the solvate compound Rb4Si9 · 5NH3, which contains fourfold negatively charged nine atom silicon clusters Si94−. Using additionally the [2.2.2] cryptand during the dissolution results in the solvate [K(2.2.2crypt)]2Si5 · 4NH3 , in which the Si52− anion is present in the crystal structure. The Si52− anion has the shape of a nearly ideal trigonal bipyramid. The starting material K6Rb6Si17 contains both Si44− and Si94− Zintl anions. In ammoniate crystal structures, Si94− anions are accessible independently of Si44− anions, and ammonia of crystallisation plays a major role in the observed crystal symmetry. For the cryptate structures of Si52− and Ge52− anions ammonia of crystallisation is obligatory despite the loss of crystal symmetry compared to the crystal structures of the heavier homologues Pb52− and Sn52−.