Models orbital model VSEPR

Figure 8.7 Diborane, BaH. (a) Contour map of pb in the plane of the terminal hydrogens, (b) Contour map of pb in the plane of the bridging hydrogens, (c) Calculated geometry, (d) Experimental geometry. (e) Interatomic H-H distances, (f) Ionic model, (g) Resonance structures, (h) Protonated doublebond model, (i) VSEPR domain model showing the two three-center, two-electron bridging domains, (j) Hybrid orbital model.

Chapters 8 and 9 are devoted to a discussion of applications of the VSEPR and LCP models, the analysis of electron density distributions to the understanding of the bonding and geometry of molecules of the main group elements, and on the relationship of these models and theories to orbital models. Chapter 8 deals with molecules of the elements of period 2 and Chapter 9 with the molecules of the main group elements of period 3 and beyond. 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

Both nitrogen atoms have steric number 4 and are sp hybridized, with H—N—H and H—N—N angles of approximately 109.5°. The extent of rotation about the N—N bond cannot be predicted from the VSEPR theory or the hybrid orbital model. The full three-dimensional structure of hydrazine is shown in Figure 6.46. 

Skill 1.3c-Predict molecular geometries using Lewis dot structures and hybridized atomic orbitals, e.g., valence shell electron pair repulsion model (VSEPR)... 

A Lewis orbital model has been used to produce useful estimates of geometries, and to describe some aspects of rearrangement dynamics for and CH3BH2. Electronic repulsions roughly parallel total energy, therefore giving some theoretical support for the VSEPR model. ... 

Each of the two sp hybrid orbitals on the metal atom may be combined with the pz orbital on one of the halogen atoms to form a two-center orbital which accommodates two electrons. See Fig. 10.3. Since bonds are stronger when the overlap between central atom and ligand atomic orbitals is large, the hybridization model, like the VSEPR model, provides an explanation for the linearity of the Be, Mg and Group 12 dihalides. 

Here we have used the hybridization model after the molecular structure had been determined experimentally. Could the hybridization model - like the VSEPR model - be used to predict the structure The answer is yes we could have begun by asking if it is possible to combine the s, Px and py orbitals to form three new hybrids which are mutually orthogonal and have the same shape. The derivation is more involved, but it can be shown that the only possible candidates are the sp hydrides that we have just described, and which form angles of 120° relative to one another. There is, however, not much reason to carry out the derivation, since the VSEPR model leads us directly and quickly to the same conclusion. 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

STRATEGY Use the VSEPR model to identify the shape of the molecule and then assign the hybridization consistent with that shape. All single bonds are cr-bonds and multiple i bonds are composed of a cr-bond and one or more TT-bonds. Because the C atom is attached to three atoms, we anticipate that its hybridization scheme is sp1 and that one unhybridized p-orbital remains. Finally, we form cr- and Tr-bonds by allowing the 1 orbitals to overlap. 

Several methods have been used for analyzing the electron density in more detail than we have done in this paper. These methods are based on different functions of the electron density and also the kinetic energy of the electrons but they are beyond the scope of this article. They include the Laplacian of the electron density ( L = - V2p) (Bader, 1990 Popelier, 2000), the electron localization function ELF (Becke Edgecombe, 1990), and the localized orbital locator LOL (Schinder Becke, 2000). These methods could usefully be presented in advanced undergraduate quantum chemistry courses and at the graduate level. They provide further understanding of the physical basis of the VSEPR model, and give a more quantitative picture of electron pair domains. 

Before investigating the qualitative concepts of the VSEPR model it is worth noting that the details of the interactions between the electron pairs have been ascribed to a size-Pauli exclusion principle result . But objects do not repel each other simply because of their sizes (i.e. interpenetrations) only if the constituents of the objects interact is any interaction possible10). If we are to use the idea of orbital size at all we must avoid the danger of contrasting a phenomenon (electron repulsion) with one of its manifestations (steric effects). The only quantitative tests which we can apply to the VSEPR model are ones based on the terms in the molecular Hamiltonian specifically, electron repulsion. 

A simple orbital pieture can be used to deseribe these distortions. The lone pair ean oeeupy either an sp hybridized orbital or an unhybridized s orbital. Around eations from the upper half of the periodie table it usually oeeupies an sp orbital where the four valence-shell eleetron pairs (three bonding and one lone pair) are arranged at the corners of a tetrahedron giving rise to three primary bonds as described by the VSEPR model. Cations from lower in the table ean also show this configuration but are sometimes found with the lone pair oeeupying a pure s orbital which can then be treated as part of the core, giving a spherieally symmetric electron density. Intermediate states of hybridization are also possible and frequently found. 

In addition to the VSEPR theory8 mentioned above, other theoretical or semiempirical approaches have addressed the problem of the positions occupied by various ligands as a function of their nature, for comparison with the numerous experimental results now available molecular orbital calculations, (four-electron, three-centre model with neglect of the P d orbitals)18,19 semiempirical calculations20,21 non-empirical calculations22,23 and hybrid orbitals24,25. 

Because of its intuitive appeal and its high degree of accuracy, the VSEPR model has been well received by inorganic chemists, but the theoretical basis has been a matter of some dispute.22 More recently, there have been strong theoretical arguments for localized, stereochemically active orbitals.22... 

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