Elusive Zintl Ions [μ‐HSi4]3− and [Si5]2− in Liquid Ammonia: Protonation States, Sites, and Bonding Situation Evaluated by NMR and Theory

Long ago Zintlsized and now analyzed: ²⁹Si and ¹H NMR experiments confirm the existence of Zintl ions [μ‐HSi₄]³⁻ and [Si₅]²⁻ in liquid ammonia. For [μ‐HSi₄]³⁻, NMR and theoretical calculations reveal a bridging hydrogen atom forming a 3c‐2e‐bond. Additionally, [Si₅]²⁻, only known from solvate crystal structures so far, was characterized by ²⁹Si NMR. Here, NBO analysis indicates the formation of three localized 3c‐2e‐bonds.

Abstract

The existence of [μ‐HSi₄]³⁻ in liquid ammonia solutions is confirmed by ¹H and ²⁹Si NMR experiments. Both NMR and quantum chemical calculations reveal that the H atom bridges two Si atoms of the [Si₄]⁴⁻ cluster, contrary to the expectation that it is located at one vertex Si of the tetrahedron. The calculations also indicate that in the formation of [μ‐HSi₄]³⁻, protonation is driven by a high charge density and an increase of electron delocalization compared to [Si₄]⁴⁻. Additionally, [Si₅]²⁻ was detected for the first time and characterized by NMR. Calculations show that it is resistant to protonation, owing to a strong charge delocalization, which is significantly reduced upon protonation. Thus, our methods reveal three silicides in liquid ammonia: unprotonated [Si₅]²⁻, terminally protonated [HSi₉]³⁻, and bridge‐protonated [μ‐HSi₄]³⁻. The protonation trend can be roughly predicted by the difference in charge delocalization between the parent compound and the product, which can be finely tuned by the presence of counter ions in solution.

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