[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 K₅SnBi₃] 9 NH₃ ( 1 ) and K₉K(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.