STABILITY OF MAIN-GROUP ELEMENT-CENTERED GOLD CLUSTER CATIONS

Relativistic electronic structure calculations have been carried out f or the main-group element-centered octahedral gold cluster cations [(L Au)(6)X(m)](m+) (with central atoms X(1) = B, X(2) = C, and X(3) = N a nd ligands, L = PH3 or P(CH3)(3)) as well as for the corresponding ser ies of four-and five-coordinate element-centered cations [(LAu)(4)X(m) ]((m-2)+) and [(LAu)(5)X(m)]((m-1)+). Geometry optimization shows that the phosphine-ligated clusters have an X-Au bond which, on the averag e, is about 4 pm larger than that of the analogous naked clusters; the corresponding force constant is concomitantly weaker. The contributio n of the ligands to the overall stability of the clusters is significa nt, as the cluster, cations are stabilized more the higher; the cluste r charge; the effect is even more pronounced for trimethylphosphine li gands. When the central atom of the naked cluster core is varied, an o pposite trend is found as the cluster stability decreases along the se ries B --> C --> N. Both effects compounded lead to a maximum of stabi lity for the cluster cations [(AuL)(4)N](+), [(AuL)(5)C](+), and [(AuL )(6)C](2+), in agreement with the experimental results. Furthermore, a ll ligated octahedral clusters are calculated to be stable with respec t to the loss of an AuL(+) moiety while the corresponding reaction lea ding to a five-coordinate cluster core is energetically feasible for t he naked metal clusters. Thus-the study of ligand-free models is not m eaningful for an analysis of the electronic structure of gold. phosphi ne compounds.