The Structure of [HSi₉ ]^3- in the Solid State and Its Unexpected Highly Dynamic Behavior in Solution

We report on the first unambiguous detection of the elusive [HSi₉ ]^3- anion in solutions of liquid ammonia by various ²⁹ Si and ¹ H NMR experiments including chemical exchange saturation transfer (CEST). The characteristic multiplicity patterns of both the ²⁹ Si and ¹ H resonances together with CEST and a partially reduced ¹ H,²⁹ Si coupling constant indicate the presence of a highly dynamic Si₈ entity and a Si-H moiety with slow proton hopping. Theoretical calculations corroborate both reorganization of Si₈ on the picosecond timescale via low vibrational modes and proton hopping. In addition, in a single-crystal X-ray study of (K(DB[18]crown-6))(K([2.2.2]crypt))₂ [HSi₉ ]⋅8.5 NH₃ , the H atom was unequivocally localized at one vertex of the basal square of the monocapped square-antiprismatic cluster. Thus experimental studies and theoretical considerations provide unprecedented insight into both the structure and the dynamic behavior of these cluster anions, which hitherto had been considered to be rigid.