We have re-investigated the structural identity of the famous gold-phosphine-halide Au:PR3:X compound having 55-69 gold atoms and core size of 1.4 nm (similar to "Schmid gold" or Nanogold®) from the viewpoint of the Superatom-Complex (SAC) model for ligand protected metal clusters, and in consideration of the ligand-adatom groups observed previously for the structurally known 39-atom cluster [Au39(PR3) 14Cl6]-1. Density functional theory is used to define the formation energy of various compositions and structures, enabling a comparison of the stability of different cluster-sizes. In agreement with the SAC model, we find a strong correlation between optimal energy and delocalized electron shell closings: The 58 electron shell closing is a driving force behind the energetics. Of all compositions studied here, the energetically best one is [Au69(PR3)20Cl12]-1 anion, which has a truncated decahedral 37-atom core encapsulated by 20 Au:PR3 and 12 Au-Cl groups. It is energetically and chemically far superior to the standard models based on Au55(PR3) 12X6. Critical comparisons are made to recent experiments (NMR and mass spectrometry).
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