Cluster stabilization

In the Rieske proteins from bci or b f complexes, a third loop ( Pro loop, part of (38-/39 containing the highly conserved sequence Gly-Pro-Ala-Pro) covers the cluster from the other side. Mutations in the Pro loop have shown that this loop is critical for cluster stability... 

The definitions of the cluster ligands of the [3Fe-4S] cluster and of the related structural features are quite useful to predict cluster types in other Fds of known sequence, as well as to determine the nature of the cluster coordinating atoms (and variability) and their control on the type and performances of the metal sites, in particular in terms of cluster stability, cluster interconversion capability, and acceptance of other metals at the cluster. 

Figure 15. Formation of Ptig clusters stabilized by trialkylaluminium.

We first prepared gold clusters stabilized by conventional water-soluble pol5mer, FVF (FVF = poly(V-vinyl-2-pyrrolidone), M = 40kDa) [21]. 

Heptanuclear Trigonal Prismatic Clusters Stabilized by Monodentate Imidazole Ligand... 

Triacontanuclear Octahedral Clusters Stabilized by Bidentate Acetate Ligand... 

Scheme 8.4 Mechanism for the hydrogenation of alkenes catalyzed by ruthenium clusters stabilized by edge-bridging diphosphine ligands (CO ligands omitted for clarity).

In particular the last observation is easily understood by the molecular orbital picture given above. The 7z>orbitals of the monomers are required for a stabilization of the otherwise anti-bonding cluster orbitals of t2 symmetry which must accept six electrons. If this mixing is prevented because these orbitals adopt the electron density of the ligands, e.g., 7z>electrons of the side-on coordinated cyclopentadienyl groups, their contribution to the cluster stability is minimized or in particular cases the formation of clusters does not occur at all. Thus, the substituents attached terminally to the clusters strongly influence their stability by the different donor or acceptor capabilities. A further effect may result from the different steric demand of the substituents which will be discussed below. 

As was shown in previous section, the many-body forces play a crucial role in metal cluster stability. So, a model potential must include many-body terms, at least 3- and, sometimes, 4-body ones. For clusters of larger size, the fitted parameters in these terms will include ( absorb ) many-body effects of higher orders. 

High-potential iron proteins, 45 313-314, 344 cluster stability, 45 324-332 function, 45 315-316 residues, 45 322-344 structure and, 45 317-322 redox properties, 45 333-344 solvent accessibility, 45 330, 332-333 source and function, 45 314-316 structure, 45 316-322 hydrogen bonding and, 45 321-322 intermolecular aggregation, 45 322 primary, 45 317-318 secondary and tertiary, 45 318-321... 

Although most of the structurally characterized gold clusters are phosphine-based systems, in recent years some studies have focused on the synthesis of gold clusters with other stabilizers coexisting with phosphines such as thiols, arsines and boranes. Certain heteroleptic gold clusters stabilized with these ligands and arylphosphines in the same molecule have been structurally characterized. 

Tetrameric silver clusters stabilized by internal ligands from NMe2 - and OMe-sub-stituted aryl groups seem to exist, too (272). Silver can also be incorporated into hexanuclear clusters of the type CUg Ag Ar4Br2 with = 0, 2, 4, 6 by reactions of [MAr]4 with MBr2 (M = Cu, Ag) in the corresponding ratio (256, 265, 272). 

Within the (M6Xi2) + (X = Cl, Br q = 2-4) cluster halides, the only series that cover all three oxidation states are the tantalum halide hydrates and the haloanions [(MsXi Xg] -. The isolation of [(Ta6Cli2)Cl2(PR3)4](PF6) (n = 1, 2) led to the first complete series of group VA cluster stabilized by an organic ligand. 

In this communication we have reported the synthesis and characterization of different types of surface modified MCM-41 type pure-silicate and alumino-silicate materials. Simultaneous incorporation of thio- and amino-propyl groups was also achieved. There is tremendous potential in this field as these materials itself can be used as supra-molecular ligand in complexation with a metal ion. They can also be potentially used as host for different nano-cluster stabilization. 

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