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 SiAu₅^- is dominated by the tetrahedrally coordinated Si motif, which can be viewed as built from the tetrahedral SiAu4-by an extra Au atom bonded to a terminal gold atom. However, SiAu₆ ^- and SiAu₇^- have quasi-planar structures, similar to those of GeAu₆^-/SnAu₆^- and GeAu₇^-/SnAu₇^-, respectively. SiAu₈^- 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 SnAu₄^-, respectively. Population of minor isomers was observed for SnAu₅^-, GeAu₆^-, SnAu₆^-, and GeAu ₈^-. The 3D to quasi-2D to 3D structural evolution for SiAu₅^- to SiAu₈^- 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). © 2009 American Chemical Society.