Metal cluster compounds characterization

As is the case for transition metal cluster compounds in general, singlecrystal X-ray diffraction is normally the only technique available for the unambiguous structural characterization of heteronuclear Group IB metal clusters. Tables I, II, and IV-XIV indicate the mixed-metal clusters containing one or more ML (M = Cu, Ag, or Au L = two-electron donor ligand) units which have been studied by X-ray crystallography. Other... 

Another technique that has recently proved to be very useful for facilitating the characterization of heteronuclear Group IB metal cluster compounds is fast atom bombardment (FAB) mass spectrometry. Although the mass spectra of some mixed-metal clusters containing ML units have... 

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen. ... 

Before discussing various high-nuclearity clusters and cluster compounds we want to address the general question of what can be defined as a metal cluster. To this end we consider two examples of low-nuclearity Au compounds. According to a widely used definition, a metal cluster is characterized by direct chemical bonds between metal atoms.Here, we want to correlate the metal-metal distance in the cluster with the strength of the metal-metal bond. 

The electronic absorption spectra (UV-Vis) of polymer-embedded metal clusters contain valuable information on their electronic structure and bonding. They illustrate the way in which metallic properties develop in clusters of large enough size. Unfortunately, little systematic study on the electronic spectra of metal cluster compounds has been done, although this approach can yield important structural information. A short description of UV-Vis spectroscopy characterization of clusters with different nuclearity follows. 

The usual chemical and spectroscopic techniques such as elemental analyses, molecular weight determinations, and infrared, NMR, and electronic spectroscopy, are frequently useful for the characterization of transition metal cluster compounds. However, x-ray crystallography and, to a lesser extent, mass spectrometry, are almost indispensable for the unambiguous identification of new metal cluster systems. Indeed, much of the rapid progress in metal cluster chemistry since the appearance of Cotton s review article in 1965 (98) can be attributed to the widespread availability of automated x-ray-diffraction equipment. This increases significantly the number and accuracy of crystal structures that can be determined with a given amount of manpower, and thus the analyses of a great many more new metal cluster crystal structures are completed each year. 

X-ray single crystal diffraction, and this method remains one of the most valuable tools for the characterization of newly synthesized metal cluster compounds. In particular, the presence of short metal atom distances, determined by X-ray diffraction, indicate the presence of metal-metal bonds in clusters. The X-ray diffraction data also indicate the identity and disposition of ligands, such as CO, which usually surround the metal cluster core. A simple metal carbonyl cluster compound is [Fe2(CO)9], (Fig. 1). 

However, great progress is being made, and metal cluster compounds are in the spotlight in the field of chemistry. Recently clusters such as [Pti9(CO)22]" have been characterized. The [Pt38(CO)44] dimer has been recently mentioned and interestingly exhibits a ccp structure found in bulk Pt. 

The interaction of butadiynediyl dimetal complexes [Fp -C -CsC-M, Fp =FeCp (CO)2, M= Fp, Rp, SiMea, Rp= RuCp(CO)2] with diiron nonacarbonyl, Fe2(CO)9, results in the formation of a mixture of products, as is also observed in the case of their interaction with organic acetylenes. Interesting polymetallic complexes, propargylidene-ketene compounds, zwitterionic cluster compounds, and pa-p -propargylidene-cyclobutene compoimds were isolated from the reaction mixtures and successfully characterized. The product distributions were found to be dependent on the metal fragment (M) at the other end of the C4 rod. The results of the reaction are described... 

The reactions of the butadiynediyldimetal(Fe, Ru) complexes with Fe2(CO)ci at room temperature afforded mixtures of products, from which three types of products, viz. the ps-acetylide cluster compound 4, the pj-ti -propargylidene-ketene compound 5 and zwitterionic cluster compound 6, were isolated. While the reaction with an excess amount of Co2(CO)g results in addition to the sterically congested Fp -C=C part [6]. The distributions of the products were dependent on the metal fragments situated at the other end of the conjugated carbon rod. The cluster compounds so obtained were characterized by spectroscopic and... 

Metal clusters in metal oxide systems have not been well-characterized or abundantly investigated up to the present time. Only isolated examples of metal-metal bonded units in oxide lattices have appeared from time to time. It will be the thesis of this presentation to show that highly unusual structures determined by strong metal-metal bonding will be found in ternary and quaternary metal oxide systems, and that opportunities abound for creative work on the synthesis, theory and structure-property relationships of such compounds. Because of the well-known correlation of d-electron population and d-orbital radial extension with metal-metal bond formation,... 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

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