Valence shell expansion

Atoms of these elements have empty J-orbitals in the valence shell. Another factor—possibly the main factor—in determining whether more atoms than allowed by the octet rule can bond to a central atom is the size of that atom. A P atom is big enough for as many as six Cl atoms to fit comfortably around it, and PC15 is a common laboratory chemical. An N atom, though, is too small, and NC15 is unknown. A compound that contains an atom with more atoms attached to it than is permitted by the octet rule is called a hypcrvalent compound. This name leaves open the question of whether the additional bonds are due to valence-shell expansion or simply to the size of the central atom. 

A hybridization scheme is adopted to match the electron arrangement of the molecule. Valence-shell expansion requires the use of d-orbitals. 

Jorgensen CK (1969) Valence-Shell Expansion Studied by Ultra-violet Spectroscopy. 6 94-... 

Lucken, E. A. C. Valence-Shell Expansion Studied by Radio-Frequency Spectroscopy. Vol. 6, pp. 1-29. 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

The immediate object of this review is the results of the application of radiofrequency spectroscopic methods to the valence-shell expansion problem but it is hoped that this will serve as an illustration of the more general one of their application to any problem in electronic structure. 

Valence-Shell Expansion Studied by Radio-Frequency Spectroscopy... 

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