METAL-METAL COORDINATION CHEMISTRY - FREE CLUSTERS OF GROUP-11 ELEMENTS WITH SODIUM

Mixed metal clusters of sodium with copper, silver, and gold were prod uced by supersonic expansions at well-defined thermodynamic conditions . Differences in the behavior toward binary alloying, cluster formatio n, and relative thermodynamic stabilities are discussed within a struc tureless, delocalized, electronic shell model and using molecular orbi tal arguments and the different strengths of the relativistic effects of the coinage metals. Free-energy changes (Delta G) of selected excha nge reactions are determined, showing the substitution of gold atoms t o be thermodynamically the most favored. This is due to the preferred s-d hybridization in gold atoms, leading to a stronger metal-metal int eraction. The measured ionization potentials reveal a distinct shell c losing at the octamer for all three heterosystems. The drop of the ion ization potential. going from the octamer to the nonamer is most prono unced for the sodium-silver heteroclusters and is almost as large as f or pure sodium clusters.