[SnBi₃]⁵⁻ —A Carbonate Analogue Comprising Exclusively Metal Atoms

The new [SnBi₃]⁵⁻ polyanion is obtained by the reaction of K₃Bi₂ with K₄Sn₉ or K₁₂Sn₁₇ in liquid ammonia. The anion is iso(valence)electronic with and structurally analogous to the carbonate ion. Despite the high negative charge of the anion, the Sn−Bi bond lengths range between single and double bonds. Quantum-chemical calculations at a DFT-PBE0/def2-TZVPP/COSMO level of theory reveal that the partial double bond character between the heavy main-group atoms Bi and Sn originates from a delocalized π-electronic system. The structure of the anion is determined by single-crystal X-ray diffraction analyses of the compounds K₅[SnBi₃] 9 NH₃ (1) and K₉[K(18-crown-6)][SnBi₃]₂⋅15 NH₃ (2). The [SnBi₃]⁵⁻ unit is the first example of a carbonate-like anion obtained from solution, and it consists exclusively of metal atoms and completes the series of metal analogues of CO and CO₂.