Capping principle

The Capping Principle has been derived for evaluating the electron count in condensed polyhedra from those of its component polyhedra. Thus, it has been established that the total electron count in a condensed polyhedron is equal to the sum of the electron count for the parent polyhedron minus the electron count characteristic of the atom, pairs of atoms, or face of atoms which are common to both polyhedra. The frontier orbitals of the capping fragment are matched in 

A classical example of correlation of structure with valence electron count is shown in Fig. 2.34. There the structures of a series of osmium clusters are systematized by using the capping principle combined with the approach of decapping (i.e. removal of cluster vertices). 

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Wade s electron-counting procedures (22) start from a closed polyhedron and require that neither removing a vertex from this polyhedron nor capping a face will alter the number of skeletal bonding orbitals. This capping principle has been demonstrated to be general (31). For... 

An alternative way of thinking about this structural type (as opposed to a confacial bioctahedron) is as an [EX6]3 octahedron in which one face of the octahedron is capped by a neutral EX3 unit. This is a somewhat contrived point of view but, in later sections, this capping principle will be quite useful in rationalizing a number of structural types. 

Of these three Principles mentioned in Section 3, the Capping Principle is the easiest to understand. The others will only be mentioned briefly here more details may be found elsewhere. It should also be mentioned that King has proposed alternative visualizations of the bonding in such systems within his graph-theoretical approach. ... 

The sputter coating of polymers (misrepresentation of the real phenomenon) is another important demonstration of how important the CAP mechanism is not only in plasma polymerization but also in other plasma processes. Thus, inadvertent plasma polymerization can take place in an attempted etching process conversely, etching can occur in an elfort to deposit a polymer by plasma polymerization. Therefore, a thorough understanding of the CAP principle seems to be important for the successful operation of any plasma process. 

The schematic diagram of CAP principle is shown in Figure 10.1. In this scheme, four major processes are necessary to complete the mass balance in the reactor (1) monomer feed in (2) ablation (3) material deposition and (4) escape from the system (pump out). The formation of reactive species is an ablation process because considerable fragmentation of the monomer (starting material) occurs in general cases. 

Figure 10.1 Schematic diagram of the Competitive Ablation and Polymerization (CAP) principle (1) dissociation (ablation) of monomer to form reactive species, (2) deposition of plasma polymer and ablation of solid including plasma polymer deposition, (3) deposition to and ablation from nonsubstrate surfaces, and (4) removal of stable molecules from the system.

Alternatively, one might explore close-packed structures of boranes. Consider, for example, the difference between deltahedral and tetra-capped octahedral BioHio structures (Fig. 4). The former is stable as the dianion (11 cluster pairs) while, based on Extended Htickel calculations (60), the latter is slightly more stable for the +6 cation (7 cluster pairs). The capping principle predicts 7 cluster pairs for capped octahedra (61). A structurally characterized transition metal analogue of the tetra-... 

Worked example 23.4 Application of Wade s rules (PSEPT) the capping principle... 

The OS7 core of Os7(CO)2i is a capped octahedron. Show that this is consistent with the PSEPT capping principle. 

Use the capping principle to account for the fact that [Osg(CO)22] has a bicapped octahedral structure. 

Boranes tend to adopt rather open structures, and there are few examples of BH units in capping positions. However, application of the capping principle does allow satisfactory rationalization of some condensed cages such as Os6(CO)i8 (24.37). 

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