Metal clusters

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Metal clusters are compounds of at least three metal atoms in which each metal atom is bonded to at least two others. A distinction is made between pure metal clusters, which are usually only stable in a matrix, and ligand-stabilized clusters . These differ from polynuclear metal complexes in that there is a metal-metal bond between the metal atoms . Clusters can contain one or more types of metal atoms. The number of bonded metal atoms ranges from three to several tens of thousands. Very large metal clusters are also referred to as nanocrystals. With an increasing number of metal atoms, the metallic character increases, with clusters of up to 55 atoms one also speaks of meta-metals .

history

Triiron dodecacarbonyl

The term cluster was coined by F. Albert Cotton in the 1960s . The investigation of metal carbonyls led to the synthesis of large carbon monoxide stabilized clusters such as [Rh 13 (CO) 24 H 3 ] 2− . Investigations by Linus Pauling showed that molybdenum chloride (MoCl 2 ) consisted of Mo 6 octahedra, which were stabilized by chloride ligands. Cotton found that rhenium chloride (ReCl 3 ) consists of Re 3 Cl 9 units. The diamagnetism of this compound indicates the presence of a Re-Re double bond.

In biology, iron-sulfur and iron-sulfur-molybdenum clusters were identified as the active centers of various proteins such as ferredoxins and nitrogenase in the 1970s .

presentation

The representation of clusters can be distinguished into that of naked, non-ligand-stabilized clusters and the ligand-stabilized clusters. The full shell clusters are of particular importance . The number of metal atoms in a full-shell cluster corresponds to the formula 10 n 2 + 2 ; they represent error-free sections from a metal grid .

Naked clusters

The representation of non-ligand-stabilized metal clusters, so-called naked clusters , is often difficult. The clusters obtained have a relatively large size distribution. Laser evaporation can be used to create bare clusters of up to 30 atoms from the metals lead and tin.

Ligand-Stabilized Clusters

The number of known ligand-stabilized clusters is relatively large. They can be produced using conventional organometallic synthesis methods, such as the photochemical abstraction of ligands. A number of compounds and ions have been used as stabilizing ligands and range from carbon monoxide (in the case of metal carbonyls ) to complex ligands such as silsesquioxanes. Examples are Fe 3 (CO) 12 , Co 4 (CO) 12 , [Pt 38 (CO) 44 ] 2− , [Cu 12 S 8 ] 4− or Au 55 [P (C 6 H 5 ) 3 ] 12 Cl 6 .

Investigation of clusters

Some elementary relationships in chemistry , such as the mode of action of a large number of catalysts or the transition between individual atoms and macroscopic matter , have not yet been fully clarified. In both examples the problem lies in the investigation of metal clusters. The reactive clusters that form the transition between atoms and matter can also provide information about the structure of possible catalytic centers. In both cases, examining clusters provides a way of obtaining results that are otherwise inaccessible. The matrix technique has proven itself for the investigation of clusters . This makes it possible to isolate and study reactive species for a longer period of time. In order to achieve an isolation with as little interaction as possible, matrix technology largely concentrates on noble gas matrices . In addition to the matrix technique, only experiments in a dilute gas phase in which the molecules are largely free of interaction are possible for investigating reactive molecules . However, matrix technology offers the essential advantage that, due to the accumulation in a matrix, spectroscopic methods such as NMR or Raman spectroscopy can be used if an investigation of certain species in the gas phase is not possible. It was only with the help of matrix technology that fundamental information about the structure and formation of alkali metal clusters was obtained. The condensation and analysis of a noble gas matrix requires a lot of equipment. The isolation in matrices that are rigid at higher temperatures could simplify the matrix technique. Working with such matrix materials requires less technical effort than when using a noble gas. Günter Schmid intensively investigated the stabilization of transition metal clusters by ligands. He succeeded in depicting gold clusters with 55 gold atoms, which are stabilized by phosphine ligands.

In the field of cluster research, there are no meaningful studies on matrices that are stable at room temperature, although theoretical considerations exist that predict a high thermal stability of clusters up to this temperature range. So far, matrices stable at room temperature have been used almost exclusively for the analysis of organic substances in solid-state matrices or organic glasses. Isolation in rigid matrices opens up a multitude of spectroscopic and preparative possibilities for all areas of chemistry. With matrices that are stable at room temperature, it should be possible to use spectroscopic methods that are available for analyzing a solid. This possibility can Inertgasmatrizes that require constant cooling, do not offer. An example of this is NMR spectroscopy, which has now become an irreplaceable analytical tool in all disciplines of chemistry. However, this method was rarely used in matrix technology. The few publications of NMR spectroscopic investigations on matrices are almost completely limited to the field of organic chemistry.

See also

literature

  • G. Schmid: Metallcluster - study objects of metal formation, chemistry in our time, 22nd year 1988, No. 3, pp. 85–92, doi : 10.1002 / ciuz.19880220303

Individual evidence

  1. ^ G. Schmid: Von Metallclustern und Clustermetallen , in: Nachrichten aus Chemie, Technik und Laboratorium , Volume 35, Issue 3, pages 249-254, March 1987.
  2. Silsesquioxanes as Ligands for Gold Clustersas (PDF; 336 kB) by G. Schmid, R. Pugina, J.-O. Malm and J.-O. Bovin in: European Journal of Inorganic Chemistry. Volume 1998, Issue 6, pp. 813-817, June 1998.

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