Molecular metal clusters

It has been argued that the behaviour of ligands on the edges and faces of metal clusters (molecular compounds based on three or more connected metals) might model the behaviour of such ligands respectively on a step or surface of a bulk metal (the cluster-surface analogy, Figure 2.19). 

Although ab initio methods are usually quite reliable, one still has to be very cautious when dealing with transition metal clusters, molecular clusters, and so on, since conventional DFT methods still contain certain approximations and might fail in distinguishing various structural isomers. Sometimes, benchmark calculations of the performance of various choices of DFT methods are needed before initiating the global search of PES, which consists of a large number of DFT calculations. 

Study hybrid metal cluster/molecular oxide systems. 

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

Wang L S and Wu H 1998 Probing the electronic structure of transition metal clusters from molecular to bulk-like using photoeieotron spectroscopy Cluster Materials, Advances In Metal and Semiconductor Clusters vo 4, ed M A Duncan (Greenwich JAI Press) p 299... 

Andres R P ef a/1996 Self-assembly of a two-dimensional superlattice of molecularly linked metal clusters Science 273 1690... 

Complexes involving larger metal clusters, eg, Au or Au2Rh, also undergo piezochromic rearrangements with rather dramatic changes in the absorption spectmm, and well-defined changes in molecular stmcture (6,7). 

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

Synthesis, molecular dynamics and reactivity of mixed-metal clusters. G. L. Geoffrey, Acc. Chem. Res., 1980,13,469-476 (29). 

Catalysis by molecular metal clusters. E. L. Muetterties andM. J. Krause, Angew. Chem., Int. Ed. Engl., 1983, 22,135-148 (106). 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

The most extensive studies on the genetics and molecular biology of CODH have been performed with the coo system of R. rubrum. A coo gene cluster contains CODH (CooS), an Fe-S electron-transfer protein (CooF), metal cluster assembly proteins (CooCTJ) (126), and... 

Ames Laboratory (Iowa State University, USA) investigating new solid state phases based on reduced rare earth halides. Since 1993, she has held a position at the University Jaume 1 of Castello (Spain) and became Associate Professor of Physical Chemistry in 1995. During the second semester of 2005, she held a visiting professor position at the Laboratory of Chemistry, Molecular Engineering and Materials of the CNRS-Universtity of Angers (France). Her research has been focussed on the chemistry of transition metal clusters with special interest in multifunctional molecular materials and the relationship between the molecular and electronic structures of these systems with their properties. She is currently coauthor of around 80 research papers on this and related topics. 

Finally, feasibility studies have clearly demonstrated that S-layer technologies have a great potential for nanopatteming of snrfaces, biological templating, and the formation of arrays of metal clusters, as required in nonlinear optics and molecular electronics. 

Oxide- and Zeolite-supported "Molecular" Metal Clusters Synthesis, Structure, Bonding, and Catalytic Properties... 

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. 

Oxide- and Zeolite-Supported Molecular Metal Clusters... 

The literature of metal-support interactions includes httle about the possible chemical bonding of metal clusters or particles to supports. Supported molecular metal clusters with carbonyl ligands removed have afforded opportunities to understand the metal-support interface in some detail, and the results provide insights into the bonding of clusters to supports that appear to be generalizable beyond the small clusters to the larger particles of conventional supported metal catalysts [6]. 

---