METAL CLUSTER COMPOUNDS AS MOLECULAR PRECURSORS FOR TAILORED METAL-CATALYSTS

Organometallic molecular cluster compounds with metal frameworks are akin to metal particles in having neighboring multimetal centers. The adjacent metal sites in polynuclear clusters make available coordination environments that cannot be realized at a single metal atom or ionic site typical of most homogeneous catalysts. Transition-metal clusters are ideal objects for the study of collective behavior in stoichiometric and catalytic reactions. In this sense, metal clusters occupy an intermediate position between molecular (homogeneous) and solid-state (classic heterogeneous) catalysis. High selectivity is achieved by homogeneous metal complexes (including metal clusters) in homogeneous solution catalysis because of the uniformity of the complexes. Heterogeneous catalysts that involve colloidal, crystalline, and supported metal particles have the advantage of being readily separable from the products, economically and by physical means. The ease of separation gives heterogeneous catalysts a practical advantage over homogeneous catalysts. The activity and selectivity of heterogeneous catalysts generally depend on the state of metal dispersion (particle size), structure (shape and morphology), metal composition, and metal-support interactions. Metal dispersion depends on the selection of the precursors and supporting materials and also on the method of preparation. © 1992, Academic Press Inc.