Metal-Metal Bonds and Clusters in Chemistry and Catalysis

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We have a dedicated site for Germany. The symposium was larger than previous IUCCP symposia since it also celebrated the 25 years that had elapsed since the initial discovery by F. Cotton and his co-workers of the existence of metal-metal quadruple bonds. Cotton's discovery demonstrated that multiple bonding in inorganic systems is not governed by the same constraints observed in organic chemistry regarding s and p orbital involvement.


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The d orbitals are involved in the multiple bonding description. The quadruple bond involves considerable d orbital overlap between adjacent metal centers. Part I of this series of papers focuses upon the impact of this discovery and describes further contributions to the development of the field.

Multiple metal-metal bonding now is known to permeate broad areas of transition metal chemistry. The understanding of metal-metal bonding that developed as a result of the discovery of multiple metal-metal bonding awakened a new chemistry involving metal clusters. Clusters were defined by Cotton to be species containing metal-metal bonding.

Clusters in catalysis therefore seemed a logical grouping of papers in this symposium. Clusters play an every increasing role in the control of chemical reactions. Part II of this book describes some of the interesting new developments in this field. In Part III the papers examine the role clusters play in describing and understanding solid state materials.

Thermochemical Aspects of Organotransition Metal Chemistry. In one study [20] an EMAC was obtained that consisted of 9 chromium atoms in a linear array with 4 ligands based on an oligo pyridine wrapped around it. In it the chromium chain contains 4 quadruple bonds. Although few metal carbonyl clusters are catalytically useful, naturally occurring Iron-sulfur proteins catalyse a variety of transformations, such as the stereo-specific isomerization of citrate to isocitrate via cis - aconitate , as required by the tricarboxylic acid cycle.

Nitrogen is reduced to ammonia at an Fe-Mo-S cluster at the heart of the enzyme nitrogenase. Hydrogenases rely on Fe 2 and NiFe clusters. Metal carbonyl cluster compounds have been evaluated as catalysts for a wide range of reactions, especially for conversions of carbon monoxide.

Introduction

The clusters Ru 3 CO 12 and Ir 4 CO 12 catalyze the Water gas shift reaction , also catalyzed by iron oxide , and Rh 6 CO 16 catalyzes the conversion of carbon monoxide into hydrocarbons, reminiscent of the Fischer-Tropsch process , also catalyzed by simple iron compounds. Some define cluster catalysis to include clusters that have only one active site on one metal atom. The definition can be further relaxed to include clusters that remain intact during at least one reaction step, and can be fragmented in all others. Metal carbonyl clusters have several properties that suggest that they may prove as useful catalysts.

The absence of large bulk phases leads to a high surface-to-volume ratio, which is advantageous in any catalyst application as this maximizes the reaction rate per unit amount of catalyst material, which also minimizes cost. In general, as the number of atoms in a metal particle decrease, their coordination number decreases, and significantly so in particles having less than atoms.

Metal clusters are sometimes characterized by a high degree of fluxionality of surface ligands and adsorbates associated with a low energy barrier to rearrangement of these species on the surface. Interconversion ligands between terminal, double-, and triply bridging sites is often facile. It has further been found that metal atoms themselves can easily migrate in or break their bonds with the cluster structure.

Although not used for any commercial process, metal carbonyl clusters have been subjected to many studies aimed at demonstrating their reactivity. Some of these examples include the following:. Species that are typical ligands for a metal cluster represent obvious reactant-catalyst combinations. It has been proposed that coordination of CO to multiple metal sites weakens the triple-bond enough to allow hydrogenation. The selectivity is heavily influenced by the particular cluster used. Steric effects are the most important consideration in many cases, however electronic effects dominate in hydrogenation reactions where one adsorbate hydrogen is relatively small.

In some cases, a metal cluster must be "activated" for catalysis by substitution of one or more ligands, such as acetonitrile.

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Metal-metal bonds and clusters in chemistry and catalysis [1990]

An example of the former is empirical packing energy calculations, where only interactions between adjacent atoms are considered. The packing potential energy can be expressed as follows:. DFT has been used more recently to study a wide variety of properties of metal clusters. However the fundamental form of the energy functional is only approximately known, and unlike other methods there is no hierarchy of approximations which allow a systematic optimization of results.

Tight bonding molecular dynamics has been used to study bond lengths, bond energies, and magnetic properties of metal clusters, however this method is less effective for clusters with less than metal atoms due to a larger influence of approximation errors for small clusters. From Wikipedia, the free encyclopedia. Main article: Metal carbonyl cluster. Introduction to cluster chemistry.

Englewood Cliffs, N. J: Prentice Hall. Seyferth Bino; M. Ardon; I. Maor; M. Kaftory; Z. Dori Bino; F. Cotton; Z. Dori; M. Kapon; D. Marler; G.

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Reisner; W. Schwotzer; M. Shaia Braunstein, L. Oro, P. Scharfe; F. Kraus; S. Stegmaier; A. Schier; T. Angewandte Chemie International Edition. Spiekermann; S. Hoffmann; T. Zubarev; Alexander I. Wang; S. Stegmaier; B. Wahl; T. Schnepf; H. Stegmaier; T. Hoffmann Fischer; et al. Bibcode : PhRvL.. Wiley-VCH: Weinheim, Science: AAAS. Nature Materials: Springer Nature. Concepts and models for characterizing homogeneous reactions catalyzed by transition metal cluster complexes.

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Physical properties of metal cluster compounds. Model systems for nanosized metal particles. Growth and Properties of Metal Clusters. Amsterdam: Elsevier Scientific Publishing Company. Synthesis of organic compounds catalyzed by transition metal clusters. Metal clusters - The relationship between molecular and crystal structure. Concepts and Models of Inorganic Chemistry third ed.

Heterometallic clusters for heterogeneous catalysis. A review". Journal of Molecular Catalysis. Studies in Surface Science and Catalysis. Activation of ruthenium clusters for use in catalysis: Approaches and problems. Computational Materials Science.

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