Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (17), 3066 - 3140
The Base Side of Gold
Positively and negatively charged
as neighbors in a crystal-lattice
Pure gold is the epitome of nobility - even in chemistry, since gold is very chemically inert. However, gold is not as inert as one would like to think. There is thus an entire series of compounds that contain gold atoms in various oxidation states. In these compounds, the gold can exist as either a positive or, as has been known for a few years, a negative ion. Amazingly, the noble metal gold has a number of similarities to the halogens (fluorine, chlorine, bromine, iodine, and astatine). It seems especially closely related to iodine: gold has an electron affinity comparable to that of iodine. Electron affinity is a measure of how strongly an atom can bind an additional electron.
Martin Jansen and Anja-Verena Mudring have now discovered another surprising parallel to the chemistry of the halogens: under certain reaction conditions an electron is transferred from one gold atom to another. Thus positively and negatively charged gold particles are formed from elemental gold, a process called disproportionation.
How do the researchers cause the gold to undergo electron transfer with its own kind? If one allows gold to react with the alkali metals cesium or rubidium and the corresponding alkali metal oxide in a specific ratio at 425°C, compounds of the type (MAu)4(M3AuO2) are formed. M represents the alkali metal, Au stands for Aurum = gold, and O is oxygen.
Structural studies have revealed that gold particles with an oxidation state of +1 are present in the resulting crystal. They are bound to oxygen - in the form of characteristic, dumbbell-shaped O-Au-O structures. This group is called aurate. Beside these, in the crystal are Au ions with a single negative charge, known as auride ions. The unusual name of this unusual class of compounds is derived from these structural features: aurideaurate.
"The newly discovered aurideaurates add to the previously small group of compounds that contain the same element in positive and negative oxidation states adjacent to each other,"? reports Jansen. "These gold compounds are amazingly stable. Despite the small distance between the Au- and the Au+ in the crystal, the oppositely charged particles don't react with each other to return to elemental gold."