[SnBi ] -A Carbonate Analogue Comprising Exclusively Metal Atoms.

The new [SnBi ] polyanion is obtained by the reaction of K Bi with K Sn or K Sn 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 K [SnBi ] 9 NH (1) and K [K(18-crown-6)][SnBi ] ⋅15 NH (2). The [SnBi ] 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 CO .