Why very small metal clusters? From a practical standpoint, industry often wants its metallic catalysts in a finely divided form so as to maximize the surface area available for a given mass of metal. Obviously, stability toward agglomeration and reactant accessibility are also necessary prerequisites for the successful performance of highly dispersed metal particle catalysts. Solid supported catalysts usually achieve this end. Moreover, such catalysts offer the advantage of convenient handling in a process system; however, technical metallic catalysts are usually ill defined and poorly understood, and better defined systems are necessary for fundamental experimental work.