VSEPR model success

The conformation of lowest energy appears to be that of C3v symmetry. Part of the experimental difficulties stems from the fact that the molecule is highly dynamic and probably passes through several conformations. In either of the two models shown in Fig. 6.13, the Xe—F bonds near Ihe lone pair appear to be somewhat lengthened and distorted away from the lone pair however, the distortion is less titan would have been expected on the basis of the VSEPR model. That the latier model correctly predicted a distortion at all at a time when others were predicting a highly symmetrical octahedral molecule (all other hexafluoride such as SF6 and UF are perfectly octahedral) is a signal success, however. 

Now we are ready to start the derivation of the intermediate scheme bridging quantum and classical descriptions of molecular PES. The basic idea underlying the whole derivation is that the experimental fact that the numerous MM models of molecular PES and the VSEPR model of stereochemistry are that successful, as reported in the literature, must have a theoretical explanation [21], The only way to obtain such an explanation is to perform a derivation departing from a certain form of the trial wave function of electrons in a molecule. QM methods employing the trial wave function of the self consistent field (or equivalently Hartree-Fock-Roothaan) approximation can hardly be used to base such a derivation upon, as these methods result in an inherently delocalized and therefore nontransferable description of the molecular electronic structure in terms of canonical MOs. Subsequent a posteriori localization... 

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. 

The VSEPR model is the least sophisticated it makes practically no use of the mathematical machinery of quantum mechanics. Its emphasis on the influence of interelectronic repulsions is in sharp contrast to the other models, in which repulsions are not explicitly mentioned except perhaps at a secondary stage to refine details of the structural prediction. Nevertheless, this model achieves considerable success and should be regarded as a useful qualitative tool in handling structural problems. It is interesting that the repulsive forces invoked in the VSEPR model may not, in fact, be simple electrostatic ones. Instead, they may be attributable to the combined effects of the orthogonality of orbitals and the Pauli exclusion principle.9... 

Assuming that the VSEPR model can be applied successfully to each of the foUowing species, determine how many different fluorine environments are present in each molecule or ion (a) [SiFg] , (b) Xep4, (c) [NF4], ... 

For many years after the classic work of Werner which laid the foundations for the correct formulation of fif-block metal complexes, it was assumed that a metal in a given oxidation state would have a fixed coordination number and geometry. In the light of the success (albeit not universal success) of the VSEPR model in predicting the shapes of molecular species of the p-block elements (see Section 2.8), we might reasonably expect the structures of the complex ions [V(OH2)6] [Mn(OH2)6] + ([Pg.620]

There have been other approaches to enhance the relative contributions of the valence shell electron density distributions. Thus, visualizing the second derivative of the electron density distribution led to success and the emerging patterns paralleled some important features predicted by the VSEPR model [17]. 

The VSEPR model is probably the most successful and the most widely used model for predicting the shapes of simple non-ionic molecules. It builds directly on the Lewis formula of the molecule, but has been influenced by quanmm mechanics in so far as the electrons are allowed to move. A succinct description of the model has been given in a recent textbook by Gillespie and Hargittai [9] ... 

The electron localization function (ELF) is another tool that has been used with considerable success in highlighting domains in p(r) of strong electron localization (Becke and Edgecombe 1990). These domains, like those defined by the maxima in VSCC, have likewise been associated with the bonding and nonbonding electron-pairs of the VSEPR model (Becke and Edgecombe 1990 Bader et al. 1996 Bader et al. 1996 Savin et al. 1997). The ELF has also been found to reveal the shell structure of an atom in a clear and faithful fashion. 

To summarize, VSEPR model has been remarkably successful in predicting the structures of main-group compounds, but the predictions remain qualitative. All attempts to quantify them were so far unsuccessfiil, as well as attempts to extend this approach to transition metals ( extended VSEPR model ) taking into account such factors as ligand-ligand repulsion and polarization of the core electron shells of the central atom [158-160], In the latter case, many other factors must be considered, such as rf-electron configuration of the central atom, competition between a and n bonding, etc. (see discussion in [154]). 

Why the VSEPR approach should be so successful has been muchly discussed whether the electron pairs are truly similar in energy and whether they repel by either simple electrostatic forces or by the Pauli Exclusion principle (i5). In his comparison of the VSEPR "points-on-a-sphere" formalism with results of Molecular Orbital computations of potential energy surfaces, Bartell concluded that "the VSEPR model somehow captures the essence of molecular behavior" (76). 

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. 

The character of a covalent bond, the main focus of this chapter, was identified by G.N. Lewis in 1916, before quantum mechanics was fully developed. Lewis s original theory was unable to account for the shapes adopted by molecules. The most elementary (but qualitatively quite successful) explanation of the shapes adopted by molecules is the valence-shell electron pair repulsion model (VSEPR model). In this model, which should be familiar from introductory chemistry courses, the shape of a molecule is ascribed to the repulsions between electron pairs in the valence shell. The purpose of this chapter is to extend these elementary arguments and to indicate some of the contributions that quantum theory has made to understanding why atoms form bonds and molecules adopt characteristic shapes. 

The Valence Shell Electron Pair Repulsion model correctly predicts the structures of main group compounds and efi transition metal compounds in most cases. The structures of less symmetric molecules can be predicted semi-quantitatively. The model is particularly successful if the coordination number (including the count of nonbonding electron pairs) does not exceed six. A peculiarity of the model is the stereochemical activity of the free electron pair. Whether the VSEPR model can be successfully extended to coordination numbers larger than 6 is the topic of the following discussion. 

One can see from Equation (4.10) that hybrid p-character (and associated X hybridization parameter) strongly affects hybrid direction and molecular shape. But what affects hybrid p-character The answer to this question gives one of the deepest insights into molecular shape, and is expressed in simple and intuitive terms by Bent s rule (cf. FcfeB, p. 13 8ff), the deeper principle that underlies success of the valence shell electron pair repulsions (VSEPR) model (Sidebar 4.3). 

Two qualitative models have been successful in accounting for many of the structural changes in sulfoxides and sulfones5. One is the Faience Shell Electron Pair Repulsion (VSEPR) theory8, while the other approach involves considerations of nonbonded ligand/ligand interactions9. 

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

---