Caesium auride

Caesium auride
Solution of CsAu(left), pure CsAu(right)
Names
IUPAC name
Caesium auride
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/Au.Cs X markN
    Key: COOMJVRPVOQALF-UHFFFAOYSA-N X markN
  • covalent form: [Cs][Au]
  • ionic form: [Cs+].[Au-]
Properties
AuCs
Molar mass 329.872022 g·mol−1
Appearance Yellow crystals
Melting point 580 °C (1,076 °F; 853 K)
reacts violently
Structure
CsCl
a = 4.24 Å
Thermochemistry
−123 kJ/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Caesium auride is the inorganic compound with the formula CsAu. It is the Cs+ salt of the unusual Au− anion.

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.

Gold has the highest electronegativity of all metals due to relativistic effects. It has been called a "pseudo-halogen".

Preparation and reactions

CsAu is obtained by heating a stoichiometric mixture of caesium and gold. Non-stoichiometric mixtures result in an alloy rather than an ionic compound. The two metallic-yellow liquids react to give a transparent yellow product.

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. In this way it splits the difference between typical ionic solids, which are insulators, and typical metallic solids, which are conductors. This semiconductivity is a relativistic effect.

The compound hydrolyzes readily on contact with water, yielding caesium hydroxide, metallic gold, and hydrogen.

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.

The solid is transparent and red.

Crystal structure

Caesium 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, as found by X-ray photoelectron spectroscopy, because gold has a much higher electronegativity than caesium.

References

  1. 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.
  2. ↑ V P Laguzov, Ya A Yukhvidin, Zh. Fiz. Khim. 44 (1970) 1827; Chem Abstr 74 (1971) 7225
  3. ↑ 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.
  4. ↑ 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.
  5. 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.
  6. 1 2 3 4 "Cesium Compounds". www.cs.rochester.edu. Retrieved 2025-07-27.
  7. 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