Clusters deltahedral

Organometallic Deltahedral Clusters of the Heavier Croup 13 Metals and More Complicated Structures Derived from Deltahedra... 

The heavier group 13 metals E = Al, Ga, In, T1 form much weaker E-H than B-H bonds. Thus, the deltahedral hydrides E H 2- remain unknown. However, hydrides of these elements are well known to be less stable, i.e., to have lower decomposition temperatures than the corresponding alkyls. Consequently, replacement of the relatively weak E-H bonds with stronger E-C bonds has been shown recently to give stable deltahedral clusters of the general type E R Z (E = Al and Ga z = 0, 1, and 2 in the known examples). 

An unusual type of deltahedral cluster found in organoindium chemistry but not in deltahedral borane chemistry is the eight-vertex In8[Si(CMe3)3]6 (see Chapter 2.3.4.2) its structure is based on a bicapped octahedron with idealized D34 sym-... 

Closed deltahedral clusters, which contain group 15 or group 16 in the skeleton, exhibit structures well-known from the borates B H 2 (Figure 3.4-1). 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

Closo, nido, arachno, hypho deltahedral clusters based on n main group vertex atoms are characterized by An + 2, An + 4, An + 6 and An + 8 total valence electrons respectively. The corresponding optimum VEC for closo and nido deltahedra... 

As with the BWH2 polyhedra, it can be shown that the optimum number of bonding electron pairs should be n + 1 for other deltahedral clusters (i.e., those having entirely triangular faces) of metal atoms M (n 5). For example, [Os5(CO),2]2" should have 12 electrons available for cluster bonding and its metal atoms are arranged in the form of a trigonal bipyramid. 

Am rfo-deltahedral cluster with n vertices is held together by (n+2) bonding electron pairs. 

Most clusters, however, caimot be described adequately in terms of two-center, two-electron bonds because the coimectivity of the vertices exceeds the number of valence orbitals that are available for bonding. Early efforts to rationalize such systems, such as Lipscomb s styx approach and Kettle s Topological Equivalent Orbital Method, are described by Mingos and Johnston. In these more difi cult cases, the simple valence-bond picture is inappropriate examples are deltahedral clusters composed of B-H vertices or conical M(CO)3 fragments, both of which usually have only three orbitals available for skeletal bonding. Theoretical models for describing these systems will be discussed in the next section. 

This approach is perhaps best illustrated by a simple example. Consider a deltahedral cluster where the vertex connectivity is greater than three, for example, BeHe ". One... 

The reason why the TSH analysis of deltahedral clusters, with n + 1 skeletal bonding orbitals, is inapplicable is that the L and L cluster orbitals are not necessarily bonding and antibonding, respectively, in three-connected clusters. This result is basically due to the low coordination number and the relatively sparse manner in which the vertices in three-connected structures cover a sphere. ... 

Negatively charged clusters of some of the p-metals and semimetals are also stable in solutions and can be crystallized from them as ionic molecular compounds. Known, at present, are a number of such ligand-free anionic clusters, called Zintl ions. Some examples are the clusters with general formulas PUy", Eg , Eg , aud Ej where Pn = P, As, Sb and E = Si, Ge, Sn, Pb. This chapter will limit its attention to the synthesis, structure, and functionalization of molecular anionic clusters of tin. Although both nine- and five-atom clusters are known (Figure 2.6.1), the focus will be predominantly on the nine-atom deltahedral clusters Sug and Sn which are the most readily prepared and most extensively studied species. 

The most stable clusters of tin, as well as of the rest of the elements in this group except carbon, are, by far, the nine-atom clusters. As mentioned above, Sng was the first deltahedral cluster to be structurally characterized. While Geg was added very soon after, nine-atom clusters of lead and silicon were found respectively 20 and 30 years later. ° Thus, the following discussion of geometry, charges, cluster-bonding, and electronic structure is valid not only for tin clusters, but also for those of silicon, germanium, and lead. 

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