Stable Au III complexes with four N-heterocyclic carbene groups can be prepared in high yield directly from KAuCl 4 .

Gold(iii) N-heterocyclic carbene (NHC) complexes of form [Au(NHC)4 Cl2 ]Cl were synthesized by reaction of KAuCl4 with bis- and tetrakis(imidazolium) salts in the presence of a mild base. Treatment of these complexes with KPF6 afforded four-coordinate AuIII complexes of form [Au(NHC)4 ](PF6 )3 . X-Ray crystallography showed the [AuIII (NHC)4 ]3+ cations in the hexafluorophosphate salts to have a square planar Au(NHC)4 moiety [AuCNHC 2.024(4)-2.082(7) Å]. In the [AuIII (NHC)4 Cl2 ]+ cations in the chloride salts, coordination about Au was tetragonally-distorted octahedral, the axial Au-Cl bonds being substantially longer [AuCl 3.148(2)-3.693(1) Å] than the equatorial Au-CNHC bonds [AuC 2.024(4)-2.082(7) Å]. NMR and conductance studies suggested that the structures of the complexes seen in the solid state persisted in DMSO solution, except in one case where a chlorido ligand dissociated from [AuIII (NHC)4 Cl2 ]+ to form [AuIII (NHC)4 Cl]2+ . The AuIII (NHC)4 unit was surprisingly robust. An AuIII complex was found to undergo H/D exchange reactions in D2 O solution at 100 °C with no signs of decomposition detectable by 1 H NMR spectroscopy. 1 H NMR studies showed that various complexes containing AuIII (NHC)4 moieties underwent little or no decomposition when heated at 120 °C in DMSO-d6 for extended periods.