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Gold(I) Cyanide

AuCN salt
Molar Mass
222.985 g/mol
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Properties

StateSolid
ColorYellow to lemon
SolubilityInsoluble in water; soluble in alkali cyanide solutions (as [Au(CN)2]-)
Melting PointDecomposes 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. OSHA PEL for cyanides as CN is 5 mg/m3 (8-hour TWA); IDLH is 25 mg/m3. 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.

Constituent Elements

Au — Gold C — Carbon N — Nitrogen

Frequently Asked Questions

What is the molar mass of gold(I) cyanide?
AuCN weighs 222.985 g/mol: gold (196.967) + carbon (12.011) + nitrogen (14.007). Gold's atomic weight has unusually low IUPAC uncertainty (±0.001) because it's monoisotopic — natural gold is 100% Au-197, so this molar mass is good to four decimal places without source-dependent variation.
Why is AuCN central to commercial gold mining?
Cyanide leaching is the only economically viable process for extracting gold from low-grade ore (1-5 g/tonne), which is what most mines actually have. Crushed ore is slurried with dilute NaCN and aerated; gold dissolves as [Au(CN)2]⁻ via the MacArthur-Forrest reaction, while silica, iron, and most other gangue minerals stay solid. The gold-bearing solution then flows through tanks of activated carbon (CIP/CIL processes) that adsorbs the dicyanoaurate, which gets stripped with hot caustic, electroplated to bullion, and refined. AuCN appears as an intermediate in some recovery and electroplating steps. The process accounts for roughly 80% of the world's roughly 3,000 tonnes annual gold production.
Why does gold(I) prefer two-coordinate linear geometry?
Gold(I) is d¹⁰, so crystal field stabilization gives no preference for any geometry — but relativistic effects do. In gold, the inner 1s electron moves at roughly 60% of the speed of light, which raises its relativistic mass and contracts the 1s orbital; this contraction propagates outward and pulls the 6s orbital in too, while the 5d orbitals (less affected by the relativistic core) expand outward. The energy gap between filled 5d and empty 6s shrinks enough that strong sd-hybridization becomes favorable, but only along a single axis (forming two sd hybrids pointing 180° apart). Two-coordinate linear coordination is therefore the thermodynamic ground-state geometry for Au(I), shared with the isoelectronic and similarly-relativistic Hg(II).