Ligand Stabilized Metal Colloids

Water soluble phosphines such as the sodium salt of sulfonated triphenylphos-phine, (m-QH4S03Na)3P, function as stabilizers for hydrosols of colloidal rhodium, prepared by hydrogen reduction of Rha3 3H20 in the presence of the surfactant. [67] The colloidal material has been described as a polyhydroxylated rhodium particle, which implies considerable oxidation of, at least, the rhodium surface to Rh(I). 

The use of phosphines as stabilizers for colloidal metals is reminiscent of the stabilization of low valent transition metals. Since the latter have found extensive application in homogeneous catalysis such ligand manipulation should be a powerful method for modifying colloidal metal catalysts as well. 

8 A Comment on Ligand Stabilized Giant Molecular Clusters and Colloidal Metal Particles 

5 nm 0.3 nm diameter could not be considered molecular. However, the following points should be considered when contemplating the nature of the giant clusters i) since the narrow size distribution of the colloidal metal particles is within the precision limits of the TEM measurements, even a truly molecular sample would have a range of apparent diameters ii) the mean diameter of 

5 nm does correspond to a five shell cubic packed cluster iii) the clusters are prepared in the absence of any severe steric constraints, such as those imposed in particle preparation in zeolite cages, and yet they do not aggregate further. All these considerations are consistent with the notion that a special stability is associated with this degree of aggregation. To describe these materials as molecules requires a rather unconventional use of the term, but perhaps no less unconventional than a description of organic polymers as molecular. Th too are polydis- 

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Schmid, G. Maihack, V. Lantermann, F., and Peschel, S., Ligand-stabilized metal clusters and colloids properties and applications, J. Chem. Soc. Dalton Trans., 589, 1996. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Even in the case of Pd" catalysts, it is not clear how different types of ligands (phosphines, phosphine oxides, amines) stabilize the active metal center in the various steps of the reactions, whether they form distinct metal complexes or whether they rather stabilize metal colloids [49]. 

Schon G and Simon U 1995 A fascinating new field in colloidal science small ligand stabilized metal clusters and their possible applications in microelectronics Colloid Poiym. Sci. 273 202... 

Nath S., Jana S., Pradhan M., and Pal T. Ligand-stabilized metal nanoparticles in organic solvent. J. Colloid Interface Sci. 341 no. 2 (2010) 333-352. 

Schon, G. and U. Simon. 1995. A fascinating new field in colloid science—small ligand-stabilized metal-clusters and possible application in microelectronics. 1. State-of-the-art. Colloid Polym. Sci. 273 (2) 101-117. 

The decomposition is significantly accelerated and the temperature of the first decomposition reaction is lowered to 120°C (Fig. 19.7). The decomposition rate is relatively low compared with other titanium-based dopants. The highest activity of a titanium catalyst used in alanate decomposition was observed for ligand-stabilized colloidal titanium metal [42]. 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

Schmid, G. Ligand-stabilized Giant Metal Clusters and Colloids. In Physics and Chemistry of Materials with Low-Dimensional Structures, Kluwer Academics The Netherlands, Longh, J. L. 1994 Vol. 18, pp 107. 

Schmid, G. and Peschel, S., Preparation and scanning probe microscopic characterization of mono-layers of ligand-stabilized transition metal clusters and colloids, New J. Chem., 22, 669, 1998. 

In contrast to standard borohydride reductive nanoparticle synthesis, we have developed an alternative strategy to amino acid encapsulated nanoparticles by utilizing a metal nanoparticle (M°-(Ligand))/metal ion (M"+) precursor redox pair with matched oxidation/reduction potentials. Simply, a metal nanoparticle such as Pt°-(Cys) acts as the principal reductant to a complimentary selected metal ion of Au + resulting in a new stabilized metal nanoparticle of Au°-(Cys) and the oxidation product of the original nanoparticle Pt"+. Malow et al. have reported a metathesis/transmetallation type reaction between a platinum colloid and a Au cyanide compound. Similarly, we employed a Pt°-(Cys)/AuCl4 pair and 0.5-2.0 equivalents of Au to Pt -(Cys). XRD analysis of the nanoparticle products revealed differences in crystallinity... 

Metal clusters and colloids cannot be isolated in an unprotected form, as coalescence processes set in immediately by contact between particles to give amorphous or polycrystalline powders. Therefore, colloids have always been used in highly diluted dispersions, in polymers or in matrices [1]. Clusters are well known as stable compounds if they are protected by a shell of appropriate ligands. To make colloids available as isolable molecules (e. g. for homogenous catalysts), one developed a method to stabilize them by a ligand shell similar to that of clusters [2-4]. Colloids thus became applicable for numerous chemical and physical investigations [2, 5]. Even bimetallic particles have been stabilized and made useful in various practical applications [5]. 

Ligand-stabilized species can be described as colloids, according to the definition given above. Colloids used in polymer matrices or other stabilizing liquid media are not considered here. Continuing the principle of ligating metal particles of cluster size, only such metal particles are taken into account which exist as individuals outside the liquids in which they are produced. 

In a recent perspective article G. Schmid et al. summarized some general phenomena and the great catalytic potential of ligand-stabilized transition-metal clusters and colloids. The catalytic properties of large ligand-stabilized palladium clusters has been described. ... 

The study of naked transition metal clusters forms the basis and reference for understanding the properties of the corresponding ligated and supported species. Most of the experimental evidence so far available deals with ligand-stabilized or supported clusters which are the species commonly encountered in colloidal solutions, catalytic materials, and cluster based nanostructured materials. Thus, systematic theoretical work on free unperturbed transition metal clusters is especially needed to provide fundamental information, since these clusters are not so easily accessible experimentally. 

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