Molecular metal carbonyl cluster

An alternative to this physical method of preparing structurally uniform metal clusters on supports involves chemistry by which molecular metal carbonyl clusters (e.g., [Rh6(CO)i6]) serve as precursors on the support. These precursors are decarbonylated with maintenance of the metal frame to give supported nanoclusters (e.g., Rh6). Advantages of this chemical preparation method are its applicability to many porous supports, such as zeolites (and not just planar surfaces) and the opportunities to use spectroscopic methods to follow the chemistry of synthesis of the precursor on the support and its subsequent decarbonylation. Zeolites, because their molecular-scale cages are part of a regular (crystalline) structure, offer the prospect of regular three-dimensional arrays of nanoclusters. 

A limitation of supported metal nanoclusters prepared from molecular metal carbonyl clusters is that, so far, clusters of only several metals (Ru, Rh, Ir, and Os) have been made in high yields (80 to 90%, with the likely impurity species being mononuclear metal complexes). However, this disadvantage is offset by the advantage of the characterizations, which show that some clusters are stable even during catalysis, at least under mild conditions. 

Ugo R, Dossi C, Psaro R (1996) Molecular metal carbonyl clusters and volatile organometallic compounds for tailored mono and bimetallic heterogeneous catalysts. 1 Mol Catal A 107 13... 

These observations on the structurally simple carbides of the early transition metals show how the strength of binding of core carbon atoms in molecular metal carbonyl clusters can in principle be estimated by comparison with metal carbides for which structural and theoretical data are available, and leads us to hope that examination of the wider body of transition metal carbides will provide relationships between the length and strength of bonds between metal atoms and octahe-drally coordinated carbon atoms that can be applied to specific molecular metal carbonyl clusters containing core carbon atoms. 

Researchers have attempted to drive off the carbonyl ligands of molecular metal carbonyl clusters in the hope of preparing naked clusters of the same nuclearity. It is now evident, although this assertion contradicts some of the claims in the literature, that most of the attempts have failed and have instead led to increases in cluster nuclearity and loss of structural simplicity. Usually, the lack of sufficient characterization has prohibited the determination of the nudearities of all the resultant spedes, and often only the larger spedes (dusters) have been detected. With the increasing availability of techniques such as high resolution transmission electron microscopy and EXAFS spectroscopy this field is expected to develop rapidly. 

LARGE MOLECULAR METAL CARBONYL CLUSTERS MODELS OF METAL PARTICLES... 

Molecular metal carbonyl clusters (hereafter referred as clusters) are small (<15 A) metal aggregates electronically saturated and stabilized by ligands and free charges [6], whereas small metal particles (hereafter referred as crystallites) are typically electronically unsaturated and stabilized either by more or less strong inter-... 

It is not our purpose to review such a wide field we focus on some more recent accomplishments in the field of large molecular metal carbonyl clusters pertaining to their structural, electronic, chemical, and magnetic behaviour, which have some relevance to the above analogies. 

Large Molecular Metal Carbonyl Clusters and Metal Lattices... 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Application of small metal particles has attracted the attention of the scientists for a long time. As early as in the seventies Turkevich already prepared mono-dispersed gold particles [19], and later, using molecular transition metal carbonyl clusters [20], the importance of small nanoparticles increased considerably. One of the crucial points is whether turnover frequency measured for a given catalytic reaction increases or decreases as the particle size is diminished. 

Fig. 1. Schematic molecular structures of the pentanuclear high nuclearity metal carbonyl clusters and of Os6(CO)is.

Fig. 8. Schematic representation of different molecular structures in nuclearity metal carbonyl clusters.

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

Molecularly or ionically dispersed metal carbonyl clusters on metal oxides have been prepared in high yields by reaction of metal carbonyl clusters with support surfaces or by syntheses on support surfaces from mononuclear precursors (Gates and Lamb, 1989 Iwasawa, 1993 Ichikawa, 1992 Gates, 1994). Synthesis of supported metal carbonyl clusters has been reviewed recently (Gates, 1995,1998), and only a few examples are included here. 

D. Braga and F. Grepioni, Molecular Self-Recognition and Crystal Building in Transition-Metal Carbonyl Clusters—The Cases of Ru3(CO)i2 and Fe3(CO)i2- Organometallics 1991, 10, 1254-1259. 

Molecular Mechanics Simulation of Ligand Structures in Transition-Metal Carbonyl Clusters. 

A. Sironi, Inorg. Chem., 31, 2467 (1992). Transition Metal Carbonyl Clusters. A Molecular. Mechanics Approach to Ligand Stereochemistry. 

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