Stability of Main-Group Element-Centered Gold Cluster Cations

Relativistic electronic structure calculations have been carried out for the main-group element-centered octahedral gold cluster cations [(LAu)₆X_m]^m+ (with central atoms X₁ = B, X₂ = C, and X₃ = N and ligands L = PH₃ or P(CH₃)₃) as well as for the corresponding series of four- and five-coordinate element-centered cations [(LAu)₄X_m] ^(m-2)+ and [(LAu)₅X_m]^(m-1)+. Geometry optimization shows that the phosphine-ligated clusters have an X-Au bond which, on the average, is about 4 pm larger than that of the analogous naked clusters; the corresponding force constant is concomitantly weaker. The contribution of the ligands to the overall stability of the clusters is significant, as the cluster cations are stabilized more the higher the cluster charge; the effect is even more pronounced for trimethylphosphine ligands. When the central atom of the naked cluster core is varied, an opposite trend is found as the cluster stability decreases along the series B →C → N. Both effects compounded lead to a maximum of stability for the cluster cations [(AuL)₄N]⁺, [(AuL)₅C]⁺, and [(AuL)₆C]²⁺, in agreement with the experimental results. Furthermore, all ligated octahedral clusters are calculated to be stable with respect to the loss of an AuL⁺ moiety while the corresponding reaction leading to a five-coordinate cluster core is energetically feasible for the naked metal clusters. Thus the study of ligand-free models is not meaningful for an analysis of the electronic structure of gold phosphine compounds.