Gold compounds are chemical substances where the metal is bonded to at least one other element. While pure gold is famous for its resistance to corrosion, its compounds demonstrate surprising chemical activity and versatility. The compounds formed are diverse because gold can exist in several oxidation states, ranging from -1 to +5. However, the chemistry of gold compounds is primarily defined by the Gold(I), or aurous, state and the Gold(III), or auric, state.
Fundamental Inorganic Gold Compounds
The most basic gold compounds involve bonding with halogens or oxygen, which form the precursors for much of the metal’s complex chemistry. Gold(I) compounds, often featuring the Au+ ion, typically adopt a linear geometry, coordinating with two atoms from other elements. A simple example is Gold(I) chloride (AuCl), which forms zigzag polymeric chains in its solid state.
In contrast, Gold(III) compounds, containing the Au3+ ion, characteristically exhibit a square planar structure. Gold(III) chloride (AuCl3) is a prominent member of this group, serving as a frequent starting material for the synthesis of more intricate gold complexes. This compound actually exists as a dimer, Au2Cl6, where two gold atoms are bridged by chlorine atoms. Gold(III) oxide (Au2O3) is another fundamental inorganic compound, notable because gold shows little natural tendency to bond with oxygen alone.
Gold Compounds Used in Medical Treatment
For decades, certain gold compounds have been used in medicine, a practice historically known as chrysotherapy. This treatment gained prominence for its role in managing chronic inflammatory diseases, particularly rheumatoid arthritis.
Modern examples include Auranofin, an orally administered Au(I) compound, and Gold Sodium Thiomalate, which is given by injection. The therapeutic action of these gold-based drugs is related to their ability to modulate the body’s overactive immune response characteristic of autoimmune conditions.
They function by interfering with the behavior of immune cells, such as monocytes and polymorphonuclear cells. The compounds are believed to reduce inflammation by altering the activity of certain enzymes, including those found in the lysosomes of cells. A key proposed mechanism involves the gold compound’s interaction with the production of reactive oxygen species (ROS) within T-cells, which are central to the immune system’s signaling pathways. Auranofin, being an organic gold compound, is absorbed orally, a difference from older, injectable gold salts that impacts its distribution and excretion within the body.
Industrial and Catalytic Applications
Gold compounds are indispensable in large-scale industrial processes, particularly in the recovery of the metal itself from low-grade ores. The most widely used process is cyanidation, which relies on gold’s ability to form a highly stable, water-soluble complex with cyanide ions.
This reaction involves potassium or sodium cyanide and oxygen, resulting in the formation of the dicyanoaurate(I) ion, Au(CN)2-, often found in the compound K[Au(CN)2]. This complex is then extracted from the crushed ore, making the recovery of even trace amounts of gold economically feasible.
In chemical manufacturing, gold compounds have emerged as highly efficient catalysts, substances that speed up chemical reactions without being consumed. Specific organogold complexes are utilized to facilitate complex syntheses that are otherwise difficult to achieve. One notable commercial application is the use of gold-based catalysts in the production of vinyl acetate monomer (VAM), a precursor for paints and adhesives. The catalytic ability of gold is often attributed to its unique electronic properties, enabling it to activate molecules like oxygen for selective reactions.