Gold(I,III) chloride explained

Gold(I,III) chloride is the inorganic compound with the chemical formula Au₄Cl₈. It is an example of a mixed valence compound as it contains gold in two oxidation states; square-planar gold(III) and almost linear gold(I). The compound, which is black, is photosensitive as well as air- and moisture-sensitive.

Synthesis

According to the procedure by Calderazzo et al., gold(I,III) chloride may be prepared by the reaction of gold(III) chloride with gold carbonyl chloride^[1] or carbon monoxide^[2] at room temperature in thionyl chloride.

Au₂(CO)Cl₄ + Au₂Cl₆ → COCl₂ + Au₄Cl₈

2 Au₂Cl₆ + 2 CO → Au₄Cl₈ + 2 COCl₂

Structure and properties

Single crystals of gold(I,III) chloride are triclinic with a P space group and consist of discrete Au₄Cl₈ molecules with idealised C_2h symmetry. Within this the Au(I) centers are linearly coordinated with a Cl-Au-Cl bond angle of 175.0° (close to the ideal value of 180°) and an average bond length of 2.30 Å. The Au(III) centers adopt a slightly irregular square-planar conformation with the Au-Cl bond lengths for bridging chlorides (2.33 Å) being slightly longer than those of terminal chlorides (2.24 Å).

Notes and References

1. Dell'Amico. Daniela Belli. Calderazzo. Fausto. Marchetti. Fabio. Merlino. Stefano. Perego. Giovanni. X-Ray crystal and molecular structure of Au₄Cl₈, the product of the reduction of Au₂Cl₆ by Au(CO)Cl. Journal of the Chemical Society, Chemical Communications. 1977. 1. 31. 10.1039/C39770000031.
2. Dell'Amico. Daniela Belli. Calderazzo. Fausto. Marchetti. Fabio. Merlino. Stefano. Synthesis and molecular structure of [Au₄Cl₈], and the isolation of [Pt(CO)Cl₅]⁻ in thionyl chloride. Journal of the Chemical Society, Dalton Transactions. 1982. 11. 2257. 10.1039/DT9820002257.