Structural Evolution of Doped Gold Clusters: MAux-(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: MAux - (M ) Si, Ge, Sn; x ) 5-8). Well-resolved hotoelectron spectra were obtained and compared to calculations at several levels of theory to identify the low-lying structures of MAu5-8 -. 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 SiAu16 -. For M ) Ge, Sn, our results show that the major isomers of GeAu5-8 - have structures similar to those of the corresponding SnAu5-8 - clusters, and they can be viewed as grown from the previously suggested square-pyramidal GeAu4 - and SnAu4 -, respectively. Population of minor isomers was observed for SnAu5 -, GeAu6 -, SnAu6 -, and GeAu8 -. The 3D to quasi-2D to 3D structural evolution for SiAu5 - to SiAu8 - and the structural convergence for MAux - (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).