Recent advances in the chemistry of gas phase transition metal clusters

A. Kaldor, D. M. Cox
1990 Pure and Applied Chemistry  
Clusters containing a well defined number of transition metal atoms have unique chemical, electronic, and magnetic properties which vary dramatically with the number of constituent atoms, the metal, and the charge on the cluster. We have been developing experimental techniques to probe these properties for gas phase clusters. This report sumnarizes and briefly reviews some of these results. INTRODUCTION Clusters containing a well defined number of transition metal atoms have unique chemical,
more » ... unique chemical, electronic, and magnetic properties which vary dramatically with the number of constituent atoms, the metal, and the charge on the cluster (Ref. 1). Compared to atomic and bulk materials our knowledge is surprisingly limited about these clusters in the 3-100 atoms size range. This i s a challenging regime from both an experimental and theoretical perspective, because this is the transition from molecular description of matter to the solid, or liquid like. The size regime is also important from a technological perspective, because this is where unique properties are exhibited by materials in applications as diverse as catalysis and "writing" metal contacts on microscopic semiconductor devices. Interest in clusters is fueled both by the potential offered by their intrinsic unique properties and by what we can learn on the atomic scale about hard to probe, complex heterogeneous systems. Honing our intuition is important since atomically rough heterogeneous systems are particularly difficult to study with atomic resolution due limitations of the available tools. Yet in this same size regime some catalysts are believed to exhibit strong size dependent behavior (Ref. Z ) , materials often are metastable and dynamic (Ref. 3), and nanoscale materials, inhomogeneous on the scale of a few atoms exhibit unique physical and chemical properties (Ref. 4). Some fundamental questions in catalysis, e.g. active sites, adatom effects, promotion, poisoning, etc. focus on effects localized on the atomic scale and these require models with similar specificity. It has been postulated that organometallic cluster compounds serve as atomic scale models for active sites in studies of surface catalysis (Ref. 5). Indeed, this approach does yield insight about the mechanism of surface reactions. Organometall ic complexes provide structural, spectroscopic and thermodynamic data base for reaction intermediates and radicals bound to surfaces. Much effort has been invested to synthesize cluster compounds with the right catalytic properties, either as homogeneous catalysts, or as supported, heterogeneous ones. A rich menu o f multi-metal cluster compounds has been made, and much has been learned about the metal -metal bonding, cluster structure, coordination, etc., but only a few multimetal centered cluster compounds have emerged as interesting, catalytically active materials (Ref. 6). This is probably due to the coordinative saturation, and the high degree of symnetry inherent in many of these complexes. Often these complexes have not survived thermal activation, 1.e. loss of ligands even when supported. We have been interested in approaching this problem from the perspective of gas phase, "naked" metal clusters. These are at the outset free of support interactions, matrix effects, and coordinatively unsaturated. The objective is to discover their physical and chemical properties as a function of cluster size and to probe how these properties change as the degree of coordination is changed, i.e. at various stages of "dressing' of the clusters with ligands of differing properties. Finally, the ultimate challenge is to deposit the unique clusters on supports and explore the properties of these new materials. Recent advances have made it possible to perform such experiments in the gas phase using most metals and with a broad range of reagents. Now one can deposit monosized clusters of many metals on a variety o f supports. trends in the field. Rules which define the chemical bonding of transition metal clusters when coordinately saturated (Ref. 7) need further development to deal with "naked' metal clusters, though they have been useful as the clusters adsorb ligands (Ref. 8). At this point we know little of In this report we plan to briefly review the emerging 79 80 A. KALDOR AND D. M. COX
doi:10.1351/pac199062010079 fatcat:dspsxwctorarnidgw7gfdrjely