Electronic Structures and Reactivity Patterns

The close structural similarity between metal clusters and elemental metals leads one to wonder at what size do metal clusters possess physicochemical properties generally associated with metals. Furthermore, given the fact that metal surfaces are important in catalysis, there is considerable interest in determining whether large transition metal clusters will be good models for chemical and physical phenomena at metal surfaces. The essential question, stated imprecisely, is how will increasing the metal-core size affect the electronic structure and reactivity patterns of transition metal cluster compounds ... 

Free alkoxide and aryloxide anions are Bronsted bases with pK values of the corresponding alcohols ranging from 5 to 20 in water. The basicity is highly dependent on the electronic properties of the alkyl or aryl moieties. For example, the pK value of hexafluoro-tert-butanol, (CF3)jMeCOH, is 9.6, which is considerably lower than the pK value of tert-butanol (19.2), but roughly the same as that of phenol (9.9). Such differences in electronic, as well as steric, environments often leads to the different structures and reactivity patterns for compounds containing similar ancillary ligands, but different alkoxides or aryloxides. 

In order to understand the physical properties and reactivity patterns of S-N compounds it is particularly instructive to compare their electronic structures with those of the analogous organic systems.On a qualitative level, the simplest comparison is that between the hypothetical HSNH radical and the ethylene molecule each of these units can be considered as the building blocks from which conjugated -S=N- or -CH=CH-systems can be constructed. To a first approximation the (j-framework of... 

As discussed in the introduction, one important role of theory in studies of reaction mechanisms in inorganic chemistry is its ability to predict or reproduce the structure of individual inorganic complexes. This is useful where the complex being studied is an experimentally well-characterized species, as calculations can provide insight into the electronic structure and the bonding patterns of the complex and help to rationalize its reactivity based on these features. It is perhaps even more valuable, though, when the focus is on a species that has not been characterized experimentally. 

The term hypervalent and the concept of hypervalency have been sharply criticized by theoretical chemists. In particular, the concept itself has been criticized by GiUespie and Silvi who, based on the analysis of electron localization functions, wrote in 2002 that as there is no fundamental difference between the bonds in hypervalent and nonhypervalent (Lewis octet) molecules there is no reason to continue to use the term hypervalent (2002CCR53). Despite aU the criticism, the term hypervalent has been overwhelmingly accepted by synthetic chemists, and the concept of hypervalency is currently widely used to describe special structural features and reactivity patterns of polycoordinated main-group compounds. 

Setting the right chemical model is essential to the computational study of chemical behaviour. This applies to problems that are associated with aU aspects of chemistry, i.e. structure, properties and reactivity patterns. In this chapter, we focus on structural and reactivity issues. Physical properties, for instance, the calculation of thermodynamic parameters (formation energies, pK, etc.) and spectroscopic properties (vibrational, electronic, NMR, etc.), present different challenges that will not be documented here. 

The study of pure and doped metal clusters has become an interesting and rapidly progressing research topic over the past decades. This is because of their structural richness, unexpected stability and reactivity patterns, the variation of these properties with respect to the number of the constituting atoms, and also because of the observed aromaticity in certain species. Similarly to an organic compound [1], the aromaticity of a metal cluster is showed by the coexistence of different properties such as equalized bond lengths and bond orders, the thermodynamic stability, specific reactivity, magnetic behavior, and the closed electronic structure. 

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