Electronic and Geometric Structure of the Cluster Compound Au₅₅[P(C₆H₅)₃]₁₂Cl₆. A Computational Study

Relativistic density functional calculations have been carried out on the model cluster Au₅₅(PH₃)₁₂Cl₆ assuming a cuboctahedral or an icosahedral Au₅₅ metal core to model the experimentally suggested cluster compound assigned as Au₅₅[P(C₆H₅)₃]₁₂Cl₆. Besides the overall shape of the metal core, the study focused on the unresolved issue at which sites the chlorine ligands are attached. The calculations reproduce characteristic interatomic distances within ∼ 2 pm, with the exception of the Au-Cl bonds. Chlorine ligands were calculated to prefer higher coordinated sites whereas the previously postulated on-top coordination at the center of the Au (100) facets of a cuboc-tahedron was found to be unstable. In fact, the present model results suggest an ensemble of several higher coordinated sites. The often assumed cuboctahedral shape of the metal cluster core was determined to be slightly less stable than an approximately icosahedral shape. The calculations allow a rationalization of the difficulties faced in experiments that attempted to discriminate cluster isomers.