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[SnBi3]5− is the all-metal ion that mimics carbonate. It contains only tin and bismuth atoms, yet has carbonate’s 24-valence-electron count, a carbonate-like threefold structure and delocalized bonding. The cluster was reported in solution in 2017, where it could be isolated only under rigorously water-free liquid-ammonia conditions.
What is the all-metal analogue of carbonate?
The ion is [SnBi3]5−, a heteroatomic Zintl polyanion made exclusively from tin (Sn) and bismuth (Bi). Kerstin Mayer, Jasmin V. Dums, Wilhelm Klein and Thomas F. Fässler reported it in Angewandte Chemie International Edition in 2017. Their paper describes [SnBi3]5− as isoelectronic with and structurally analogous to carbonate, and as the first carbonate-like anion obtained from solution that consists only of metal atoms.
“All-metal” does not mean that the isolated material contains no other elements at all. The anion itself contains only Sn and Bi; its laboratory salts contain potassium counter-ions and ammonia molecules of crystallization. The formula’s 5− charge is balanced by those counter-ions in the solid compounds.
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Carbonate is CO32−. Counting one carbon atom (four valence electrons), three oxygen atoms (18) and two electrons supplied by the 2− charge gives 24 valence electrons. For [SnBi3]5−, tin contributes four valence electrons, the three bismuth atoms contribute 15, and the 5− charge contributes five more: 4 + 15 + 5 = 24.
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That is an isoelectronic relationship: the two ions have the same total number of valence electrons even though one is built from a light non-metal and oxygen while the other uses heavy main-group metals. Equal electron counts do not make the species chemically identical, but they provide a reason to look for related bonding and geometry.
How [SnBi3]5− reproduces carbonate’s bonding pattern
Carbonate is commonly represented by resonance structures in which a C–O double bond appears in different positions. The actual ion has a delocalized π system rather than one permanently localized double bond. The tin–bismuth cluster shows an analogous delocalization.
Single-crystal X-ray diffraction found that the Sn–Bi distances are between values expected for conventional single and double bonds. That intermediate length is consistent with partial multiple-bond character shared around the cluster, rather than three independent single bonds. Density-functional calculations at the PBE0/def2-TZVPP/COSMO level attribute this behavior to a delocalized π-electron system.
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The reported structure has a carbonate-like three-coordinate arrangement of one central tin atom and three bismuth atoms. The heavy atoms are much larger and more polarizable than carbon and oxygen, so the analogy is structural and electronic—not a claim that the two ions have identical reactivity, polarity or stability.
What “mimic” means here
- Same electron total: both ions contain 24 valence electrons.
- Related geometry: [SnBi3]5− adopts the carbonate-like cluster shape reported by crystallography.
- Delocalized bonding: Sn–Bi bonds have lengths intermediate between ordinary single- and double-bond expectations, consistent with π-electron delocalization.
- Different chemistry: the charge, constituent elements, solvent compatibility and hydrolytic stability are not those of carbonate.
How the tin–bismuth carbonate analogue is made
The synthesis uses preformed polyanionic bismuth and tin clusters rather than an aqueous reaction. The researchers reacted K3Bi2 with either K4Sn9 or K12Sn17 in liquid ammonia. The Technical University of Munich describes the starting materials as deeply colored solutions of polyanionic bismuthides and stannides.
- Prepare the metal-cluster solutions. Potassium bismuthide K3Bi2 and a potassium stannide, K4Sn9 or K12Sn17, are used as the metal-rich precursors.
- Use liquid ammonia as the reaction medium. The reported chemistry takes place in liquid NH3, not water or a conventional oxygen-containing solvent.
- Combine the bismuthide and stannide solutions. Their reaction produces the [SnBi3]5− polyanion in solution under the strongly reducing, metal-cluster conditions.
- Crystallize a potassium salt. Single-crystal X-ray studies characterized K5[SnBi3]·9NH3 and K9[K(18-crown-6)][SnBi3]2·15NH3. An institutional account also reports isolation of K5SnBi3·9NH3 as the solvated potassium salt.
- Keep the material rigorously dry. The isolated cluster decomposes readily when exposed to traces of water, so the reaction, crystallization and storage require exclusion of moisture.
The formulas show why the isolated products are salts rather than a bottle of free [SnBi3]5−. Five potassium ions balance one 5− cluster in K5[SnBi3]·9NH3. In the second crystallographically characterized compound, a potassium ion associated with 18-crown-6 accompanies two [SnBi3]5− units and the corresponding potassium counter-ions. The ammonia molecules indicated after the middle dot are part of the crystal lattice as solvates.
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How it compares with ordinary carbonate
| Feature | Carbonate, CO32− | All-metal analogue, [SnBi3]5− |
|---|---|---|
| Atoms in the anion | One carbon and three oxygen atoms | One tin and three bismuth atoms |
| Valence-electron count | 24 | 24 |
| Formal charge | 2− | 5− |
| Structural relationship | Trigonal carbonate framework with delocalized bonding | Carbonate-like three-coordinate cluster; crystallography finds delocalized Sn–Bi bonding |
| How the reported species is obtained | The available data does not specify a preparation for ordinary carbonate | K3Bi2 plus K4Sn9 or K12Sn17 in liquid ammonia |
| Moisture behavior of the species discussed here | Not the subject of the 2017 metal-cluster report | Decomposes readily in the presence of traces of water |
| Demonstrated application | Not evaluated in this comparison | No practical, industrial or biological application was reported |
The table’s key point is that “analogue” refers to a matched electronic and structural motif, not to interchangeability. Carbonate chemistry is familiar in ordinary oxygen-containing environments; [SnBi3]5− is a highly charged, moisture-sensitive cluster that exists under specialized nonaqueous conditions.
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Is [SnBi3]5− stable in water?
No. The Technical University of Munich release and the publication record both warn that traces of water cause ready decomposition of the highly charged ion. It can persist in liquid-ammonia solution and be captured in ammonia-solvated potassium crystals, but that is very different from being stable in aqueous solution.
What moisture sensitivity means experimentally
- Glassware, transfer equipment and solvents must be kept rigorously dry.
- Liquid ammonia is essential to the reported preparation; replacing it with water is not a compatible variation.
- Crystals containing ammonia of crystallization must be protected from humid air during handling and characterization.
- Loss of the intended cluster after accidental water exposure should be treated as decomposition, not as a reversible dissolution step.
The available report does not provide a water-tolerance threshold, a shelf-life measurement or a recovery procedure after hydrolysis. It therefore would be misleading to assign a specific lifetime or claim that the ion can be handled under ordinary laboratory atmosphere.
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Why this discovery matters
The work extends a sequence of metal-only analogues of carbon monoxide and carbon dioxide to a carbonate-like species. Earlier metal clusters could reproduce motifs associated with smaller carbon oxides; [SnBi3]5− shows that the carbonate electron count and resonance-style bonding can also be recreated using only heavy main-group metals.
This is a fundamental result in inorganic and main-group chemistry. It tests how far familiar bonding models can be transferred from carbon–oxygen compounds to large, polarizable atoms. The intermediate Sn–Bi bond lengths and calculated delocalized π system provide a concrete structural example rather than an analogy based only on a formula.
What the discovery does not establish
- It is not a water-stable replacement for carbonate.
- It is not reported as a reagent for carbon capture, batteries, catalysis, medicine or manufacturing.
- It does not demonstrate a commercial process or a device.
- It does not show that other tin–bismuth compositions will have the same structure or stability.
The 2017 study established synthesis, crystallographic characterization and a calculated explanation for the bonding. It did not report a practical use, biological role or industrial scale-up.
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How to read the formula and charge
The square brackets in [SnBi3]5− identify a discrete polyanion: one tin atom bonded within a cluster to three bismuth atoms, with an overall charge of −5. The superscript charge belongs to the entire four-atom unit, not to bismuth alone. In a salt such as K5[SnBi3]·9NH3, five K+ ions provide charge balance, while nine ammonia molecules occupy solvation sites in the crystal.
This distinction matters when comparing the ion with carbonate. Carbonate’s 2− charge and oxygen atoms produce a familiar distribution of charge and reactivity. The metal-only analogue reaches the same 24-electron total by combining heavier group-14 and group-15 atoms with a larger negative charge. Electron counting explains the analogy; it does not erase the chemical consequences of that charge.
Quick Recap
Key facts at a glance
- Identity: heteroatomic Zintl polyanion [SnBi3]5−.
- Elements in the anion: tin and bismuth only.
- Electron count: 24 valence electrons, matching carbonate.
- Discovery: reported by Mayer, Dums, Klein and Fässler in 2017.
- Precursors: K3Bi2 with K4Sn9 or K12Sn17.
- Solvent: liquid ammonia.
- Characterized salts: K5[SnBi3]·9NH3 and K9[K(18-crown-6)][SnBi3]2·15NH3.
- Bonding: Sn–Bi distances between single- and double-bond expectations, explained by a delocalized π system.
- Water compatibility: poor; traces of water cause ready decomposition.
- Known use: none reported beyond its significance as a fundamental inorganic-chemistry discovery.
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