4 Au + 8 NaCN + O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH
This is an oxidation-reduction (redox) reaction:
8 CII + 8 e- → 8 CI (reduction)
4 Au0 - 12 e- → 4 AuIII (oxidation)
2 O0 + 4 e- → 2 O-II (reduction)
NaCN is an oxidizing agent, Au is a reducing agent, O2 is an oxidizing agent.