A REEXAMINATION OF THE I.S. RESULTS FOR THE CLUSTER COMPOUND AU55(PPH3)12CL6

The observed low I.S. values [1] for the four distinct gold sites in the cluster compound Au55(PPh3)12Cl6 have been explained in terms of a decrease of the 6s electron density at the nucleus, due either to a lattice expansion or to a delocalization of 6s electrons over the ligand shell surrounding the gold core. Recent EXAFS measurements [2] indicate a single average distance between the gold atoms, about 3.5% less than for bulk gold. This not only excludes the lattice expansion hypothesis, it effectively increases the density of 6s electrons, making an explanation for the observed I.S. even more difficult. For an understanding of the I.S. values it is necessary to reconsider the probable occupation of the 5d orbitals within the framework provided by the XPS surface atom core level shifts developed by Citrin and Wertheim [3]. Partial confirmation has been found in preliminary XPS results [5]. The consequences for the f-factor and specific heat results [1] will also be examined. © 1990 J.C. Baltzer A.G. Scientific Publishing Company.