Transition metal carbonyl clusters

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. 

Similar experiments on a large number of transition metal carbonyls have shown that this process favors dissociation to and detection of metal clusters or atoms. Since most metal-(CO)n photofragments are themselves subject to efficient dissociation, MPI experiments do not identify the primary photoproducts. This situation contrasts sharply with electron impact ionization where the parent ion is usually formed and daughter ions are seen as a result of parent ion fragmentation. Figure 4 shows the electron impact mass spectrum of Mn2(C0) Q (33). for comparison with the MPI mass spectrum of Figure 3. 

The idea (50, 5/) of dual coordination of CO implies the presence of two coordination centers in a Fischer-Tropsch catalyst system, i.e., a carbonyl carbon coordinating center, Ma, and a carbonyl oxygen coordinating center, M6 (14). It is this concept which has led at least two groups to examine transition metal carbonyl cluster compounds as homogeneous Fischer-Tropsch catalysts. 

Johnson, B. F. G., Stereochemistry of Transition Metal Carbonyl Clusters, 12, 253. 

Transition-metal carbonyls, 16 58 Transition metal catalysts, 20 151-152 Transition-metal-catalyzed microwave-assisted reactions, 16 552 Transition metal-catalyzed reactions in ionic liquids, 26 878-897 Transition-metal clusters, structure of,... 

Coordination chemistry of ER The monomeric fragments E-R are isolobal to carbon monoxide, and many complexes analogous to transition metal carbonyls have been synthesized (41 to 43, see Figure 2.3-7) [68], In most cases these reactions started with those clusters which have a high tendency to dissociate and to form monomers, such as pentamethylcyclopentadienylaluminum(I) or the alkylgal-lium(I) or alkylindium(I) derivatives. Often the products are isostructural to the respective metal carbonyls, but exceptions are the gallium compounds 44 and 45. 

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

Figure 6.40 Typical metal-atom clusters found in transition-metal carbonyls. For clarity, the carbonyl groups are not shown. Open circles denote carbon atoms in the cluster compound. (After Edwards Sienko, 1983.)...

Metal carbonyl anions react with main group halides and oxides to yield a number of main-group transition-metal carbonyl complexes in good yields. These complexes serve as starting materials for a number of higher nuclearity cluster complexes. 

Of the linear clusters, the organogermanium-substituted transition metal carbonyl compounds are the simplest. In every case, the coordination at germanium is tetrahedral, while the transition element retains the geometry of the parent carbonyl compound (50,108,119,155, 157, 181). The Ge—M bond is almost without exception shorter than the sum of the Ge—M covalent radii (Table VI), which is cited as evidence for (d — d)-n multiple bonding. 

The carbide-centered polynuclear transition-metal carbonyl clusters exhibit a rich variety of structures. A common feature to this class of carbide complexes is that the naked carbon is wholly or partially enclosed in a metal cage composed of homo/hetero metal atoms, and there is also a subclass that can be considered as tetra-metal-substituted methanes. The earliest known compound of this kind is FesC(CO)i5, in which the carbon atom is located at the center... 

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. 

P. J. Dyson and J. S. Mclndoe, Transition Metal Carbonyl Cluster Chemistry, Gordon and Breach, Amsterdam, 2000. 

Transition Metal Carbonyls From Small Molecules to Giant Clusters... 

G. Longoni, C. Femoni, M. C. Iapalucci, and P. Zanello, Electron-sink Features of Homoleptic Transition-metal Carbonyl Clusters, in Metal Clusters in Chemistry (Eds. P. Braunstein, L. A. Oro, and P. R. Raithby, Wiley-YCH, Weinheim, 1999, Yol. 2, Chap. 3.9). 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

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. 

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