Gold(I) Cyanide
Properties
| State | Solid |
| Color | Yellow to lemon |
| Solubility | Insoluble in water; soluble in alkali cyanide solutions (as [Au(CN)2]-) |
| Melting Point | Decomposes above 600 °C |
About Gold(I) Cyanide
Gold(I) cyanide is a yellow-to-lemon crystalline solid built from infinite linear -Au-C≡N-Au-C≡N- chains, with each Au(I) coordinated to two cyanides at exactly 180°. That two-coordinate linear geometry isn't a coincidence — it's what every Au(I) compound does, and it traces back to relativistic contraction of the 6s orbital. In gold the 6s electron is moving at roughly 60% of the speed of light, which contracts the orbital, makes it more tightly bound, and enables strong sd-hybridization along a single axis. The result: Au(I) coordinates linearly with two ligands, just like its isoelectronic cousin Hg(II). AuCN is essentially insoluble in pure water (the chains pack tightly with significant Au-Au aurophilic interactions, around 3.4 Å Au-Au separations) but dissolves readily in alkali cyanide solution to form the [Au(CN)2]⁻ complex with formation constant K ≈ 10³⁹ — one of the most thermodynamically stable transition metal complexes known. That stability is what powers the MacArthur-Forrest cyanide leaching process, the chemistry that recovers about 80% of the gold mined globally every year (around 2,500 tonnes): finely ground ore gets stirred with 0.05% NaCN under mild aeration, and the reaction 4 Au + 8 CN⁻ + O2 + 2 H2O → 4 [Au(CN)2]⁻ + 4 OH⁻ pulls gold into solution as the dicyanoaurate anion, which is then recovered onto activated carbon or by zinc cementation. Industrially, AuCN itself is the operational gold source for hard-gold electroplating baths used to plate corrosion-resistant gold layers onto electronic connectors, edge-card fingers, and bonding pads.
Where you'll encounter it
If you've ever held a circuit board, plugged a USB cable into a laptop, or looked at the gold contacts on a SIM card, the 0.1-0.5 micron gold layer on those contacts was electroplated from a cyanide bath — and the gold in that bath traces back to ore that was leached with the same Au(CN)2⁻ chemistry that AuCN dissolution exemplifies. In a PCB-finishing line, the electroless nickel + immersion gold (ENIG) process drops a thin protective Au layer on copper pads from a KAu(CN)2 bath buffered around pH 6, with AuCN as the active species in solution. In a heap-leach operation in Nevada or Western Australia, crushed ore is sprinkled with 0.05% NaCN and left for weeks while gravity pulls the [Au(CN)2]⁻-rich pregnant solution into a collection pond — the same coordination chemistry that makes AuCN dissolve in cyanide also runs the entire gold mining industry.
Common Uses
- Active component in alkaline gold-cyanide electroplating baths for connector pads
- Intermediate in MacArthur-Forrest cyanide leaching of gold ore
- Source of [Au(CN)2]⁻ for hard-gold deposits on edge-card fingers
- Reagent in carbon-in-pulp gold recovery process refinements
- Catalyst precursor for gold-mediated cyanation of aryl halides
- Source compound for organogold(I) chemistry research
- Standard reference material in gold assay laboratories
Safety Information
Extremely toxic. GHS: Acute toxicity oral Category 2 (H300, fatal if swallowed) — lethal oral dose under 50 mg for an adult. Reacts with any acid (including weak acids like CO2-saturated water and stomach HCl) to release HCN gas, which blocks mitochondrial cytochrome c oxidase at the binuclear Cu/Fe center and causes cellular hypoxia within minutes. Airborne cyanide exposure limits are low, and the margin between the working limit and an immediately dangerous concentration is narrow. Handle exclusively in a ducted fume hood, never in the same lab space as acids, with cyanide-specific PPE (nitrile + chemical apron + face shield) and an antidote kit (amyl nitrite pearls, hydroxocobalamin) within reach. Cyanide-specific waste disposal is required (alkaline hypochlorite oxidation to OCN⁻ then CO2 + N2). Do not store with acids, ammonium salts, or any oxidizers.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.