Role of the Central Gold Atom in Ligand-Protected Biicosahedral Au24 and Au25 Clusters

The crystal structures of the ligand-protected clusters [Au-24(PPh3)(10)(SC2H4Ph)(5)Cl-2](+) and [Au-25(PPh3)(10)(SC2H4Ph)(5)Cl-2](2+) have been elucidated recently, and they comprise the same biicosahedral structural motif for the Au core. The only difference is the central Au atom joining two icosahedra which is absent in the Au-24 cluster. On the basis of density functional simulations, we have evaluated the structural, electronic, optical, and vibrational properties of the clusters in question with a full presentation for the thiolate and phosphine side groups. Our spherical harmonics analysis of the electronic structure shows that the chemical stability of both clusters can be understood based on an 8 + 8 electron superatom model, where each icosahedron represents a closed-shell superatom of its own with a 1S(2)1P(6) electronic configuration. Similar optical absorption and circular dichroism spectra are computed for the two cluster sizes. The hollow site in Au-24 makes the cluster more rigid as the associated icosahedral strain in the Au core is released. This results in a significant shortening of interplanar Au-Au distances and increases the breathing mode frequencies of the individual Au icosahedra.