Ligand-stabilized metal clusters: Reinvestigation of the structure of ''Au-55[P(C6H5)(3)](12)Cl-6''

Ligand-stabilized gold clusters may serve as model catalysts, and the packing of their gold atoms is therefore of great interest. The cluster of the stoichiometry AU(55)[P(C6H5)(3)](12)Cl-6 is thought to be the smallest known defined compound in which the gold maintains a cubic dense packed metallic (ccp) state. We have now reinvestigated the structure of these particles with a combination of complementary methods (XRS, cryoHRTEM, analytical ultracentrifugation, and thermoanalytical measurements). The results demonstrate that the observed particles are heterogeneous in size and structure. The size distribution is found to be continuous, with a maximum corresponding to the diameter of a hypothetical Au-55 cluster. Only an insignificant fraction of the cluster cores have a ccp arrangement of their gold atoms. We also show that upon storage in air the cluster shell undergoes a slow chemical decomposition, followed by an aggregation of the cluster cores. It is concluded that the cluster preparation does not consist of stable, well-defined small particles of metallic gold protected by a ligand shell.