Polyhedral molecules and ions

Polyhedral molecules and ions Cyclic molecules and ions Infinite linear molecules and ions Structures based on 3-connected nets the plane 6-gon net 3D nets 

Structures based on 4-connected nets the plane 4-gon net the diamond net 

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Structural studies have now been made of a number of polyhedral molecules and ions, and of these the largest class comprises the tetrahedral complexes. 

Fig. 1 Examples of polyhedral cage molecules and metalloid clusters (first row) compared to related Zintl ions and intermetalloid clusters that are obtained from the reaction of Zintl ions (second row)...

The topological transition (Fig. 1) from polymeric solids in compounds containing linked polyhedral homoatomic clusters to discrete molecular (soluble) clusters can be conveniently studied by using as examples compounds rich in elements of the main-groups 13 to 15. It is possible for main-group elements in the middle of the periodic table to form homoatomic molecules or ions with localized homonuclear 2c-2e bonds. At higher valence-electron concentrations, fewer bonding and more free-electron pairs are formed. As electron deficiency increases, however, the formation of delocalized bonds becomes necessary, a situation typical of elements on the left of the periodic table. 

Derive the polyhedral electron count for each of the following species and use this number to predict the structure of the molecule or ion. 

A METAL CLUSTER compound usually consists of a polyhedral array of metal atoms that are held together by metal-metal bonds. This metal cluster is surrounded by ligands that may be small molecules or ions, such as CO or Cl . An important tool in the development of metal cluster chemistry is single-crystal X-ray diffraction, which is used to determine the arrangement of the atoms in metal clusters. The stmctures illustrated in this section were determined by X-ray diffraction. It is important to note that the formation of cluster compounds is not limited to the metal elements. Nonmetal clusters are also known such as P4 and Ceo-... 

Clathrasils, which have polyhedral cavities, but with windows that are too small to allow the passage of other molecules, so that enclosed ions or foreign molecules cannot escape. Examples are ultramarines and melanophlogite (Si02)46-8(N2, C02, CH4). 

Molecules with multiple bonds between metal atoms often have structures with beautiful and highly symmetrical polyhedral shapes. The square prismatic [Re2Cl8]2 ion, shown in Figure 3-29, played an important role in the history of the discovery of metal-metal... 

The polyhedral carboranes may be considered as formally (not prepara-tively) derived from the B H2- ions on the basis that the CH group is isoelec-tronic and isostructural with, and may thus replace, the BH group. Thus, two such replacements lead to neutral molecules of general formula B 2C2H . All such carboranes from n = 5 to n= 12 are known, either as such or, for n = 8-10, as C,C-dimethyl derivatives. In all cases, isomeric forms of these carboranes are possible and in several instances two or more isomers have actually been isolated. 

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