VSEPR theory

The generalizations of the VSEPR model are useful, but there are hmitations to its use. In this section, we give examples that illustrate some problems. The isoelectronic species IF7 and [TeFy] are predicted by VSEPR theory to be pentagonal bipyramidal and this is observed. However, electron diffraction data for IF7 and X-ray diffraction data for [Me4N][TeF7] reveal that the equatorial F atoms are not coplanar, a result that cannot be predicted by the VSEPR model. Moreover, in IF7, the I-F x and I-F q distances are 179 and 186 pm respectively, and in [TeF7], the Te-Fju bond distance is 179 pm and the Te-Fgq distances lie in the range 183 to 190 pm. 

It is important to note that whereas VSEPR theory may be applicable to p-block species, it is not appropriate for those of the rf-block (see Chapters 19-23). 

In a square planar species such as [ICLt] or [PtC ] (1-22), the four Cl atoms are equivalent. Similarly, in [PtCl3(PMe3)] (1.23), there is only one possible arrangement of the groups around the square planar Pt(II) centre. (The use of arrows or lines to depict bonds in coordination compounds is discussed in Section 6.11.) 

If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 

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The shapes of covalent compounds are determined by the tendency for bonding pairs to be as far apart as possible whilst lone pairs have a greater effect than bonding pairs (VSEPR theory). 

VSEPR theory See valency, theory of. vulcanite See ebonite. 

VSEPR model, the dihalides of Be and Mg and the heavier halides of Ca and Sr are essentially linear. However, the other dihalides are appreciably bent, e.g. Cap2 145°, Srp2 -- 120°, Bap2 108° SrCl2 - 130°, BaCh - 115° BaBri -115° Bah 105°. The uncertainties on these bond angles are often quite large ( 10°) and the molecules are rather flexible, but there seems little doubt that the equilibrium geometry is substantially non-linear. This has been interpreted in terms of sd (rather than sp) hybridization or by a suitable id hoc modification of the VSEPR theory. ... 

Vinyl triflones 636 Vitamins, synthesis of 833 VSEPR theory 34... 

STRATEGY The existence of residual entropy at T = 0 suggests that the molecules are disordered. From the shape of the molecule (which can be obtained by using VSEPR theory), we need to determine how many orientations, W, it is likely to be able to adopt in a crystal then we can use the Boltzmann formula to see whether that number of orientations leads to the observed value of S. 

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

The other approach to molecular geometry is the valence shell electron-pair repulsion (VSEPR) theory. This theory holds that... 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

What structures will the following molecules have according to VSEPR theory ... 

Table 1.4 Shapes of Molecules and Ions from VSEPR Theory...

When VSEPR theory is used to predict molecular geometries, double and triple bonds are treated identically to single bonds as a single electron group, i.e. as a single place where you can find electrons. 

According to VSEPR theory, the most stable arrangement of the three lone pairs of electrons would be in the equatorial position, as shown in (1), where they would be less crowded. Therefore, a linear structure is the correct molecular geometry of the molecule. 

Use the VSEPR theory to work out the shapes of the following molecules or ions. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

Note In VSEPR theory the term bond pair is used for a single bond, a double bond, or a triple bond, even though a single bond consists of one pair of electrons, a double bond two pairs of electrons, and a triple bond three pairs of electrons. To avoid any confusion between the number of electron pairs actually involved in the bonding to a central atom, and the number of atoms bonded to that central atom, we shall occasionally use the term ligand" to indicate an atom or a group of atoms attached to the central atom. 

The result here is quite satisfactory because XeF4 does in fact exhibit square planar geometry. It is worth noting, however, that a square planar shape for XeF4 is also predicted by VSEPR theory. Despite the fact that the molecular orbital method has made some inroads as of late, VSEPR is still the best approach available for rationalizing the molecular geometries of noble gas compounds. 

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

In using the VSEPR theory to determine the molecular geometry, start first with the electron group geometry, make the nonbonding electrons mentally invisible and then describe what remains. 

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