Abstract
We have re-investigated the structural identity of the famous gold-phosphine-halide Au:PR(3):X compound having 55-69 gold atoms and core size of 1.4 nm (similar to "Schmid gold" or Nanogold(R)) 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 [Au(39)(PR(3))(14)Cl(6)](-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 [Au(69)(PR(3))(20)Cl(12)](-1) anion, which has a truncated decahedral 37-atom core encapsulated by 20 Au:PR(3) and 12 Au-Cl groups. It is energetically and chemically far superior to the standard models based on Au(55)(PR(3))(12)X(6). Critical comparisons are made to recent experiments (NMR and mass spectrometry).