Structural evolution of doped gold clusters: MAu(x)(-) (M = Si, Ge, Sn; x = 5-8)

We report a joint experimental and theoretical study on the structures of a series of gold clusters doped with a group-14 atom: MAu(x)(-) (M = Si, Ge, Sn; x = 5-8). Well-resolved photoelectron spectra were obtained and compared to calculations at several levels of theory to identify the low-lying structures of MAu(5-8)(-). We found that the structure of SiAu(5)(-) is dominated by the tetrahedrally coordinated Si motif, which can be viewed as built from the tetrahedral SiAu(4)(-) by an extra Au atom bonded to a terminal gold atom. However, SiAu(6)(-) and SiAu(7)(-) have quasi-planar structures, similar to those of GeAu(6)(-)/SnAu(6)(-) and GeAu(7)(-)/SnAu(7)(-), respectively. SiAu(8)(-) again has a tetrahedrally coordinated Si structure, which displays a structural motif of a dangling Au-Si unit sitting on a gold cluster surface, resembling that of the larger Si-doped gold cluster SiAu(16)(-). For M = Ge, Sn, our results show that the major isomers of GeAu(5-8)(-) have structures similar to those of the corresponding SnAu(5-8)(-) clusters, and they can be viewed as grown from the previously suggested square-pyramidal GeAu(4)(-) and SnAu(4)(-), respectively. Population of minor isomers was observed for SnAu(5)(-), GeAu(6)(-), SnAu(6)(-), and GeAu(8)(-). The 3D to quasi-2D to 3D structural evolution for SiAu(5)(-) to SiAu(8)(-) and the structural convergence for MAu(x)(-) (M = Si, Ge, Sn) at x = 6, 7 manifest competitions between the tendency of forming molecule-like structures around the group-14 dopant (optimizing M-Au interactions) and the strong tendency of forming planar structures for small gold anion clusters (optimizing Au-Au interactions).