Cluster chemistry

Metal cluster compounds are of at least three metal atoms in each metal atom is bonded to at least two other. A distinction between pure metal clusters, most of which are in a matrix stable and ligand-stabilized clusters. At this the difference with polynuclear metal complex is that between the metal atoms present, a metal -metal bond. Cluster may contain one or more types of metal atoms. The number of bound metal atoms ranges from three to several tens of thousands. One speaks in very large metal clusters also of nanocrystals. As the number of metal atoms of the metallic character increases in clusters up to 55 atoms is also called meta- metals.

History

The term cluster was coined in the 1960s by F. Albert Cotton. The investigation of metal carbonyls led to the synthesis of large carbon monoxide stabilized clusters such as [ RH13 (CO) 24H3 ] 2 -. Investigations of Linus Pauling showed that molybdenum chloride ( MoCl2 ) consisted of Mo6 octahedra, which were stabilized by chloride ligands. Cotton found that rhenium ( ReCl3 ) consisted of Re3Cl9 units. The diamagnetism of this compound suggests the presence of a Re-Re double bond.

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

Representation

The representation of clusters can be distinguished by the naked, non- ligand-stabilized clusters and the ligand-stabilized clusters. A particular importance are the full- shell cluster. The number of metal atoms in a full shell cluster is of the formula 10 n 2 2; they provide error-free extracts from a metal grid dar.

Naked cluster

The representation of non- ligand-stabilized metal clusters, so-called naked clusters, often is difficult. Cluster obtained have a relatively large size distribution. By laser - evaporation of lead and tin naked clusters with up to 30 atoms can be made from metals.

Ligand-stabilized cluster

The number of known ligand-stabilized clusters is relatively large. They can be prepared by conventional organometallic synthetic methods, for example by abstraction of photchemische ligand. The stabilizing ligand of a number of compounds and ions have been used, ranging from carbon monoxide (with metal carbonyls ) to complex ligands such as silsesquioxanes. Examples are Fe 3 (CO) 12, Co4 (CO) 12, [ Pt38 (CO) 44 ] 2 - [ Cu12S8 ] 4 - or Au55 [P (C6H5 ) 3] 12Cl6.

Investigation of clusters

Some elementary relations in chemistry, such as the operation of a variety of catalysts or the transition between single atoms and macroscopic matter are not fully clarified. In both examples, the problem lies in the investigation of metal clusters. The reactive cluster, form the transition between atoms and matter can also allow insights into the structure of possible catalytic sites. In both cases, the examination of clusters provides a possibility to obtain results that are not otherwise accessible. For the investigation of clusters, the matrix technique has proven useful. This makes it possible to isolate reactive species for a longer period and to investigate. To achieve change effects as possible isolation, the matrix technique largely focused on Edelgasmatrizes. In addition to the matrix technique only experiments in dilute gas phase to the study of reactive molecules possible in the molecules to largely non-interacting free. However, the Matrix technology offers the significant advantage that spectroscopic methods such as NMR or Raman spectroscopy can be applied by the accumulation in a matrix, where an examination of certain species in the gas phase is not possible. So it was not until the matrix technique to obtain basic information on the structure and formation of alkali metal clusters. The condensation and analysis of a rare gas matrix requires high effort. The insulation in rigid matrices at higher temperatures could simplify the matrix technique. Is for working with such matrix materials compared to using an inert gas, a lower technical effort required. Günter Schmid studied intensively the stabilization of transition metal clusters by ligand. Thus he became the representation of gold clusters with 55 gold atoms which are stabilized by phosphine ligands.

In the field of cluster research, there is no meaningful studies on room-temperature stable matrices, although theoretical considerations exist that predict a high thermal stability of clusters up in this temperature range. So far the room temperature stable matrices were used almost exclusively for the analysis of organic substances in Festkörpermatrizes or organic glasses. This insulation in rigid matrices opens up a variety of spectroscopic and preparative options for all areas of chemistry. With room-temperature stable matrices, it should be possible to use spectroscopic methods available for the analysis of a solid body available. This possibility can Inertgasmatrizes that require constant cooling, do not bid. An example of this is the NMR spectroscopy, which is now in all disciplines of chemistry is an indispensable analysis tool. However, this method has been rarely used in the matrix technique. The few publications of NMR spectroscopic studies on matrices are limited almost entirely to the field of organic chemistry.