Metal metalloid clusters

In multi-component liquids, stabilization of the liquid is revealed by the formation of eutectics where the freezing temperature is suppressed. In such liquids, the atomic species (say A and B) are not distributed at random. There are more associated AB pairs (or other clusters) than expected for a random distribution. As a result in binary metal-metalloid alloys, such as Fe-B, the low melting-point eutectics occur at preferential compositions. The most common of these is at about 17 at. % B, or an atom ratio of one B for five Fe atoms (Gilman, 1978). This suggests that clusters of metal atoms surrounding metalloid atoms form (trigonal bipyramids). These probably share corners, edges, and faces. 

The clusters described so far have in common, that the number of metal atoms is less or equal than the number of substituents. However, there is a still growing number of both neutral and anionic clusters in which the number of metal atoms is larger than the number of substituents. As a consequence, these metal-rich clusters contain naked metal centers which are only bonded to other metal centers. Schnockel referred these ones to as metalloid clusters. Several metalloid A1 and Ga clusters were prepared by standard salt elimination reactions using metastable solutions of metal subhalides MX (M = Al, Ga X = Cl, Br) as well as solutions of Gal. Since the metal subhalides were found to play the key role for the successful synthesis of this particular class of compounds, they will be discussed first. (For excellent review articles see Refs 273 and 274.)... 

Metastable solutions of GaX (X = C1, Br) as well as conventionally prepared Gal react with lithium or sodium organometallics in standard salt metathesis reactions with formation of M2R4 (Section 3.07.4.1) as well as neutral and anionic clusters of the type MnRm]x m > n) (chapter 4.1). Moreover, metalloid clusters [MnRm]x (m < n), which feature different types of metallic core structures, have been obtained. Their formation strongly depends on the reaction conditions, in particular the reaction temperature, and the (donor) solvent. 

A metal atom cluster as defined by Cotton [1] is still a very broad term, because non-metal atoms can also be part of the cluster core. In this chapter mainly two types of metal atom clusters are presented the polyborane analogous polyhedral and the metalloid clusters E Rr of group 13 elements E. The structures and bonding of the polyhedral clusters with n < r are similar to those in the well-known polyboranes. 

In a metalloid cluster [2] more metal-metal bonds than metal-ligand bonds are involved, which means n > r. The largest structurally characterized compounds of this type contain 77 A1 or 84 Ga atoms, respectively [3, 4], Metal-metal bonds dominate these clusters and the framework of the resulting metal-metal bonds exhibits a geometry similar to the bulk metal itself. With respect to the Greek word ei8o< (idea, prototype) the suffix -oid indicates that the bulk metal element is actually visible in the metal atom core of the metalloid or more generally, elementoid clusters. 

We will first comment on metal-metal bonds (see Section 2.3.2) and then discuss (see Section 2.3.3) the polyhedral clusters [6], followed by the second central subject, the metalloid clusters in Section 2.3.4 (for recently published reviews see e.g., Refs. [7-11]). 

This section will focus on homonuclear neutral or anionic clusters of the elements aluminum, gallium, indium, and thallium, which have an equal number of cluster atoms and substituents. Thus, they may clearly be distinguished from the metalloid clusters described below, which in some cases have structures closely related to the allotropes of the elements and in which the number of the cluster atoms exceeds the number of substituents. The compounds described here possess only a single non-centered shell of metal atoms. With few exceptions, their structures resemble those of the well-known deltahedral boron compounds such as B4(CMe3)4 [30], B9CI9 [31] or [B H ]2 [32]. The oxidation numbers of the elements in these... 

The principle and the significance of metalloid clusters for the understanding of the formation of metals are made clear by the two largest A1 clusters 61 and 63, which have almost the same size as the 69 and 77 A1 atoms and 18 and 20 N(SiMe3) groups [3, 91]. In both cases the A1 atoms are arranged in shells (Fig-... 

High Nuclearity Metal Carbonyl Clusters, 14, 285 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of -Complexes, 1, 239 Insertion Reactions of Compounds of Metals and Metalloids, 5, 225 Insertion Reactions of Transition Metal-Carbon Bonded Compounds I. Carbon Monoxide Insertion, 11, 87... 

The term metalloid cluster is used to describe a multinuclear molecular species in which the metal atoms exhibit closest packing (and hence delocalized inter-metallic interactions) like that in bulk metal, and the metal-metal contacts outnumber the peripheral metal-ligand contacts. Most examples are found in the field of precious-metal cluster chemistry. In recent years, an increasing number of cluster species of group 13 elements have been synthesized with cores... 

Metastable solutions of the monohalides AIX and GaX (X = Cl, Br, I) have been prepared using this technique, and oligomeric species (MX) -Em containing a variety of donors (E) have been crystallised. The solutions disproportionate to the trihalide and the metal (equation 2) when warmed to temperatures in the range -40 to 4-50 °C, depending on the halide, the donor, and the concentration. Species with oxidation states both higher and lower than +1 (i.e. on the path to both disproportionation products) have been isolated. The reduced species (0 < Nox < 1) are discussed in the section on metalloid clusters, and the monohahdes and more oxidized species (1 < Nox < 3) are discussed here. A sonochemical synthesis of a subvalent galhum species, possibly Gal, has also been developed. ... 

Careful control of these conditions allowed the isolation of clusters that form on the path to the completed precipitation of the metal. They have been termed metalloid clusters, referring to the fact that the core arrangement of the cluster atoms resembles that in the bulk metals. (This use of the term metalloid should not be confused with its use to refer to elements such as silicon, germanium, and arsenic, which lie on the metal/nonmetal border within the periodic table.) The clusters have the general formnla where n > m, and... 

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