Journal of Molecular Catalysis, 82 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ (1993) 195-209 Elsevier Science Publishers B.V., Amsterdam 195 Ml27 Ligands on clusters - adsorbates on surfaces zyxwvutsrqponmlkjihgfedcbaZYXWVU Martin Moskovits Department of Chemistry, University of Toronto, Toronto M5S lA1, Canada zyxwvutsrqponmlkjihgfe Abstract The similarities and differences between molecules adsorbed on surfaces and ligands bonded to metal clusters are considered. Although important insights obtained in one of these fields can often be applied to the other, there are several properties of surfaces which cannot be understood easily in terms of cluster analogues. Contrariwise, small metal clusters manifest a range of unique properties varying with cluster size that have no easy counterparts in surface science. Key words: adsorbates; clusters; ligands; surfaces Introduction There are three branches of physics and chemistry that seek answers to closely related questions but state these in very different languages and ap- proaches. These are the disciplines of cluster chemistry and surface chemistry. The reason that I refer to them as three rather than two disciplines is that cluster chemistry has taken two paths. The more traditional one seeks to syn- thesize soluble metal clusters stabilized by ligands. Hundreds of such com- pounds have been produced over the past two decades largely through the work of Chini et al. [ 1] and Johnson et al. [ 21. Currently these compounds include such beautiful structures as that of [Rh15 ( CO)28C2] - [ 31. Larger cluster com- pounds are continuously being synthesized so that the field of cluster synthesis is merging seamlessly with that of ligand-stabilized colloid chemistry [4-61 where the goal is to stabilize colloidal metal particles against aggregation and flocculation by attaching ligands, usually long-chain species, to the surface that then keep themselves apart by repulsive steric interactions. Currently, species that are intermediate between true compounds and colloids have been prepared with metal cores that are usually closed shell cub-octahedral crystal- lites. The number of metal atoms, N,, in such structures form the series: 13, 55,147,309,561,923... or in general N ,=1+ ; (lOn2+2) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA N=l 0304-5102/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.