Valence electrons VSEPR model application

Both the three-center bond model and the correlation diagram treatment, as just outlined, omit all central-atom orbitals except the s and p orbitals of the valence shell. Indeed the three-center bond model neglects even the s orbital except as a storage place for one electron pair. They can be described as very restricted or incomplete MO treatments. They are also inexact, even within their self-imposed limits, since numerical accuracy is neither sought nor obtained in their usual applications. It would not, of course, be sensible to strive for numerical precision after such sweeping assumptions have been made at the outset. On the other hand, the hybridization or directed valence treatment assumes very full involvement of outer d orbitals whenever more than four pairs of electrons must be accommodated. This extreme assumption is also unlikely to be accurate. Finally, the VSEPR model resorts to a simple electrostatic model, which, however successful it may be, can scarcely be taken literally. 

In Appendix 2 is outlined the most popular and successful simple model for predicting molecular geometry of main group compounds, the valence shell electron pair repulsion (VSEPR) model. However, alongside it are presented the results of some detailed calculations which prompt the comment the VSEPR model usually makes correct predictions, but there is no simple reason why . The problem of the bonding in transition metal complexes will be the subject of models presented in Chapters 6, 7 and 10 this last chapter reviews the current situation. At this point it is sufficient to comment that the most useful applications of current simple theory are those that start with the observed structure and work from there. In the opinion of the author, the general answer to the question posed at the head of this section is that we really do not know. 

Once computed on a 3D grid from a given ab initio wave function, the ELF function can be partitioned into an intuitive chemical scheme [30], Indeed, core regions, denoted C(X), can be determined for any atom, as well as valence regions associated to lone pairs, denoted V(X), and to chemical bonds (V(X,Y)). These ELF regions, the so-called basins (denoted 2), match closely the domains of Gillespie s VSEPR (Valence Shell Electron Pair Repulsion) model. Details about the ELF function and its applications can be found in a recent review paper [31],... 

Electron density maps based on theoretical calculations (methods in parentheses) are given in [22] (SCF-MO [23] also for the highest occupied MO 5ai), in [24] (SCF and Cl also for the three valence orbitals), in [10] (SCF-Xa-SW for the valence orbitals and the total valence shell), in [25] (SCF and SCGF [self-consistent group function]), and in [26] (united atom). The Laplacian V p of the charge density p showed four local concentrations of electronic charge in the valence shell of the central P atom in accordance with the VSEPR (valence shell electron pair repulsion) model [27] for this latter model and its application to PH3, see [28 to 31]. 

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