Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.[2]
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 Solution of CsAu(left), pure CsAu(right) | |
| Names | |
|---|---|
| IUPAC name
Caesium auride | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
PubChem CID |
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| Properties | |
| AuCs | |
| Molar mass | 329.872022 g·mol−1 |
| Appearance | Yellow crystals |
| Melting point | 580 °C (1,076 °F; 853 K)[1] |
| reacts violently | |
| Structure | |
| CsCl | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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When molten, it is one of the very few liquids other than typical salts (composed of a metal and a nonmetal) in which mobile ions drive conductivity.[3]
Gold has the highest electronegativity of all metals due to relativistic effects.[4] It has been called a "pseudo-halogen".[4]
Preparation and reactions
editCsAu is obtained by heating a stoichiometric mixture of caesium and gold.[5] Non-stoichiometric mixtures result in an alloy rather than an ionic compound.[5] The two metallic-yellow liquids react to give a transparent yellow product.[6]
Despite being a compound of two metals, CsAu lacks metallic properties since it is a salt with localized charges; it instead behaves as a semiconductor with band gap 2.6 eV.[4] In this way it splits the difference between typical ionic solids, which are insulators, and typical metallic solids, which are conductors.[5] This semiconductivity is a relativistic effect.[4]
The compound hydrolyzes readily on contact with water, yielding caesium hydroxide, metallic gold, and hydrogen.[6]
- 2 CsAu + 2 H2O → 2 CsOH + 2 Au + H2
The solution in liquid ammonia is brown, and the ammonia adduct CsAu·NH3 is blue; the latter has ammonia molecules intercalated between layers of the CsAu crystal parallel to the (110) plane. Solutions undergo metathesis with tetramethylammonium loaded ion exchange resin to give tetramethylammonium auride.[6]
The solid is transparent and red.[4]
Crystal structure
editCaesium auride has a cubic lattice structure of the CsCl type. Each caesium atom is octahedrally coordinated with 8 gold atoms, and vice versa. The lattice constant at ambient conditions is approximately 4.24 Å, close to that of CsCl but slightly larger due to the larger Au−
ionic radius compared to Cl−
. The bonding is predominantly ionic,[5] as found by X-ray photoelectron spectroscopy, because gold has a much higher electronegativity than caesium.
References
edit- 1 2 Kienast, Gerhard; Verma, Jitendra; Klemm, Wilhelm (June 1961). "Das Verhalten der Alkalimetalle zu Kupfer, Silber und Gold". Zeitschrift für anorganische und allgemeine Chemie (in German). 310 (3): 143–169. doi:10.1002/zaac.19613100304.
- ↑ Peer, William J.; Lagowski, J. J. (1978). "Metal-Ammonia Solutions. 11. Au−, a Solvated Transition Metal Anion". J. Am. Chem. Soc. 100: 6260–6261. doi:10.1021/ja00487a064.
- ↑ Schmutzler, R. W.; Hoshino, H.; Fischer, R.; Hensel, F. (February 1976). "Nonelectronic Electrical Transport in Liquid CsAu". Berichte der Bunsengesellschaft für physikalische Chemie. 80 (2): 107–113. doi:10.1002/bbpc.19760800203. ISSN 0005-9021.
- 1 2 3 4 5 Norrby, Lars J. (February 1991). "Why is mercury liquid? Or, why do relativistic effects not get into chemistry textbooks?". Journal of Chemical Education. 68 (2): 110. Bibcode:1991JChEd..68..110N. doi:10.1021/ED068P110.
- 1 2 3 4 "Cesium Compounds". www.cs.rochester.edu. Retrieved 2025-07-27.
- 1 2 3 Jansen, Martin (2005-11-30). "Effects of relativistic motion of electrons on the chemistry of gold and platinum". Solid State Sciences. 7 (12): 1464–1474. Bibcode:2005SSSci...7.1464J. doi:10.1016/j.solidstatesciences.2005.06.015.
Further reading
edit- Jansen, Martin (2008). "The chemistry of gold as an anion". Chemical Society Reviews. 37 (9): 1826–1835. doi:10.1039/B708844M. PMID 18762832.—includes photograph of the compound.