VSEPR scheme

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. 

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],... 

The VSEPR notation for the Cl2F+ ion is AX2E3. According to Table 11.1, molecules of this type exhibit an angular molecular geometry. Our next task is to select a hybridization scheme that is consistent with the predicted shape. It turns out that the only way we can end up with a tetrahedral array of electron groups is if the central chlorine atom is sp3 hybridized. In this scheme, two of the sp3 hybrid orbitals are filled, while the remaining two are half occupied. 

Although the discussions of the preceding molecules have been couched in valence bond terms (Lewis structures, hybridization, etc.), recall that the criterion for molecular shape (rule 2 above) was that the cr bonds of the central atom should be allowed to gel as far from each other as possible 2 at 180°. 3 at 120°, 4 at 109.5°, etc. This is (he heart of the VSEPR method of predicting molecular structures, and is, indeed, independent of valence bond hybridization schemes, although it is most readily applied in a VB context. 

The heavier atoms of Group 13 appear not to form p -p bonds. Five- and six-coordinate structures (schemes (4) and (5)) are almost entirely restricted to the heavier atoms, although at least one complex containing five-coordinate boron is known. According to VSEPR theory, scheme (4) structures should be trigonal bipyramidal. However, InClf- and TlClf are found to be square pyramidal in crystalline solids. As noted in Section 8.2, this shape (also adopted by MnClf-) is possibly favoured by crystal packing requirements. 

MOTIVATION. MOLECULAR MECHANICS AND ADDITIVE SCHEMES. STEREOCHEMISTRY AND VSEPR THEORY... 

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... 

It should be mentioned that the irregularities in bond angles caused by VSEPR are typically only a few degrees. Qualitatively, the various hybridization schemes correctly predict the overall structure. VSEPR is, however, a very useful tool for predicting further details of molecular structure, and it will be applied many times in later chapters. 

A second problem occurs as a result of the possible stereochemical non-activity of the lone pair of electrons associated with the Group 15 element, and this is a problem common to the heavier elements of the neighbouring main groups in the Periodic Table. The VSEPR approach to molecular structure of the compounds of these heavier elements is tantamount to saying that d orbitals become part of any hybridization scheme and all the valence electrons are stereochemically active. 

In all five of these hybrid orbital schemes, the use of hybridisation is only to give an improved directional overlap of orbitals to form two electron pair covalent bonds. Hybridisation does not determine the basic stereochemistry. This must still be determined by VSEPR theory and only then can hybridisation schemes be invoked to describe, more effectively, the covalent bonding present. These hybridisation schemes may equally be applied to cations and anions. The NH4 cation and BF4" anion have already been shown to involve a tetrahedral stereochemistry (Figure 6.4, examples 3 and 4) consequently the bonding in both ions may be described as involving sp hybridisation. 

It is important to understand the relationship between hybridization and the VSEPR model. We use hybridization to describe the bonding scheme only when the arrangement of electron pairs has been predicted using VSEPR. If the VSEPR model predicts a tetrahedral arrangement of electron pairs, then we assume that one s and three p orbitals are hybridized to form four sp hybrid orbitals. The following are examples of other types of hybridization. 

Use VSEPR theory to account for the structure of NH3, and suggest an appropriate hybridization scheme for the N atom. 

It is possible to extend the VSEPR rules to repulsions between bond pairs on adjacent atoms. The extension is straightforward the best structures would be the ones that place these bonds as far apart as possible. As an example, consider the already familiar molecule ethane, H3C—CH3. Since each carbon is surrounded by four electron pairs, it will assume a geometry derived from a tetrahedron, with angles of 109°, as shown in Scheme 7.8a. 

The major repulsive mechanism between electron pairs around a given atom is a quantum mechanical (QM) effect, which we alluded to in Section 1.R.3 as the exclusion principle. According to this principle, if two electrons occupy the same space, they must possess different spin directions (Scheme 1.R.1). Indeed, all the electron pairs, be they bonds or lone pairs, are net spin-less, involving two opposite-spin electrons. Since there are only two directions of the spin property, this necessarily means that two electron pairs cannot occupy the same space. If they do, there will be two electrons of the one spin type (ft) and two others with the other spin type (f f). This is excluded by the principle the exclusion actually means that this situation raises the energy very much, and hence, molecules avoid the situation as much as possible. The consequence of this is that electron pairs around a given atom will be distanced maximally in space to lower the molecular energy. This is the basis of the VSEPR rules. 

---