Transition metal clusters reactivity

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Figure Cl. 1.3. Relative reactivity of transition-metal clusters with H2 (full curves, log scale) and tire promotion...

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Concelcao J, Laaksonen R T, Wang L S, Guo T, Nordlander P and Smalley R E 1995 Photoelectron spectroscopy of transition metal clusters correlation of valence electronic structure to reactivity Rhys. Rev. B 51 4668... 

Preliminary indications are quite clear that coordin-atively unsaturated transition metal clusters are reactive, and, importantly, reactive in a controlled way. This chemistry is likely to include RC-H insertion, arom-atization etc., with the chemistry tending to produce... 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

The political justification for transition metal cluster chemistry is the assumption that clusters are models in which metallic properties may be more easily studied than in the metals themselves. These properties include electronic phenomena such as color and conductivities as well as surface phenomena, such as atom arrangements and catalytic activities. Thus, there are two main lines of cluster research. The more academic line leads to the search for new types of clusters and their structure and bonding, whereas the more practical line leads to the investigation of reactivities with the hope that clusters may open catalytic pathways that neither plain metals nor mononuclear catalysts can provide. The interdependence of both lines is obvious. 

Another quite different area where ECP s have proven to be very useful for the development of transition metal cluster models. By using a very simplified description of the metal atoms, where all electrons including the d-electrons are considered as core, certain properties of the solid material such as chemisorption on metal surfaces or the reactivity of metal clusters has been studied theoretically with considerable success. 

Novel, monomeric heteroleptic derivatives of divalent Ge 40 and divalent Sn 41 have been prepared and characterized by single crystal X-ray diffraction <19990M389>. Heteroatoms in these germylene and stannylene species are not formally part of a four-membered ring however, crystal structures point to the azametallacyclobutane structures formed due to the N —> M coordination. The heteroleptic nature of 40 and 41 gives rise to an interesting reactivity in their coupling with transition metal clusters. 

These studies are excellent examples illustrating how the structure of the reactive transition metal cluster is probed directly under catalytic reaction conditions. From these data, correlations can be drawn regarding... 

This article is concerned with one specific aspect of cluster organometallic chemistry, and describes the synthesis, characterization, structure, and reactivity of transition metal clusters containing alkyne, or alkyne-derived ligands. Alkynes display a diverse reactivity in their reactions with carbonyl clusters, and exhibit a wider range of coordination modes than any other simple, unsaturated molecule. It is this compelling diversity that has prompted the authors to undertake this review. 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards Hg for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. 

Of particular interest are the predicted magnetic moments and the question of whether or not isomers of transition metal clusters can be separated using inhomogeneous magnetic fields. To date, cluster isomers have only been detected via their different chemical reactivity (70-73). One would expect abnormally large magnetic moments for the Ih clusters if they had unusually high density of states at the Fermi level, as has been postulated for aluminum (74)-... 

The close structural similarity between metal clusters and elemental metals leads one to wonder at what size do metal clusters possess physicochemical properties generally associated with metals. Furthermore, given the fact that metal surfaces are important in catalysis, there is considerable interest in determining whether large transition metal clusters will be good models for chemical and physical phenomena at metal surfaces. The essential question, stated imprecisely, is how will increasing the metal-core size affect the electronic structure and reactivity patterns of transition metal cluster compounds ... 

The focus of this paper is experimental work on the chemical properties of neutral transition metal clusters. The outline is as follows. We first discuss in some detail the techniques used to generate neutral gas-phase clusters. Next the known physical properties of metal clusters are summarized. This is followed by a discussion of the definition of chemical reactivity in the context of the cluster experiments. Finally, several examples of specific reactions are presented and an electronic model is proposed which can explain many of the more striking observations. Results from recent cluster ion reaction studies... 

The chemisorption of CO onto 12 different transition metal clusters °° containing more than a few atoms is facile and exhibits little evidence of the dramatic size-sensitive behavior observed for chemisorption of Hj or of N2, even though Nj and CO are isoelectronic. Whether reactions are observed for the atom and the smaller clusters depends on the metal. Similarly the chemisorption reactions of CO with the cluster ions Nb and COn " exhibit little size-selective behavior. Chemisorption is faster onto the cluster ions than onto the respective neutral clusters. Aluminum is the only metal examined thus far for which CO chemisorption is significantly cluster-size sensitive, with Alg being the most reactive cluster. " ... 

As you become more familiar with transition metal clusters (no nonmetals in the framework) you will come to associate closo structures with numbers of electrons. A trimer will have 48 electrons, a tetrahedron will have 60 electrons, a trigonal bipyramid will have 72 electrons, and an octahedron will have 86. Some care is required, however, as can be illustrated with Os3H2(CO),o. An electron count gives us 46 electrons rather than 48. If, however, we allow for one Os—Os double bond, the electron count is as expected. In accord with this expectation, one osmium-osmium bond is found to be shorter than the other two and the complex shows the reactivity expected for an unsaturated complex. 

Pt and Pd Carbonyl Complexes. Prom reactivity measurement with carbon monoxide, Cox et al. [18] determined the relative CO bonding strength to transition metal clusters ... 

The first observations of an increase of the reactivity between gases in the presence of a metal were made, almost one hundred years ago, by chemists like Davy or Thenard. This pioneer work led Berzelius and Mitscherlich to define the concept of decomposition of species by contact under a catalytic force . Since then much work has been done in order to understand the behavior of these small transition metal clusters which are able to promote some chemical reactions. In fact, catalysis has become a field of enormous economical interest. It spans a wide variety of areas from oil reforming to the preparation of synthetic fibers or fertilizers. Theoretical research as well as chemical engineering are therefore deeply involved and many current studies deal with the knowledge of such materials. 

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