3.4 VSEPR Theory and LCP Theory

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3.4.1 VSEPR Theory

I VSEPR Theory: A model that predicts the geometry from the electron pairs that surround the center of the molecule.

1. Adopts a molecular geometry that minimizes electron repulsions between electron pairs.

2. works best for simple halides of the p-block elements

3. its biggest flaw is that it does not consider the steric factors of the molecule, such as the size of its substituents.

II Four conditions determine the geometry of a molecule in the VSEPR model:

1. Each valence shell electron pair of the central atom E in $EX_{n}$ is stereochemically significant (contributes to the geometry of the overall molecule).

   1. both bonding (single, double, triple) and lone pairs will determine the overall geometry of the molecule in an arrangement that minimizes repulsions.

2. Electron-electron repulsions decrease in the sequence of: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair.

   1. To minimize repulsions, lone pair-lone pair angles will be maximized.

3. Bonding pair-bonding pair repulsions also decrease in the sequence of: triple bond-single bond > double bond-single bond > single bond-single bond.

4. Repulsions between the bonding pairs in EXn depend on the difference in electronegativities of E and X.

   1. Electron-electron repulsions are weaker if the electron density of the E-X bond is drawn away from the central atom (electronegative substituent)

   2. Conversely, electron-electron repulsions are stronger if the electron density of the E-X bond is closer towards the central atom (electropositive substituent)

III The approximated angles by the VSEPR model are most accurate when the substituents (X) are identical with no lone pairs. Refer to common geometries (see “AXE method”)

1. Distortion may occur if the substituents are different or presence of lone pairs.

2. stereochemical inert-pair effect: the tendency of s-valence electrons adopting a non-bonding role

   1. This effect is mostly found in 7-coordinate species such as $[SeCl_{6}]^{2-}$ or $[TeCl_{6}]^{2-}$ that adopt octahedral structures

   2. n-coordinate species refer to n pairs of electrons of the center atom.

IV The VSEPR model should NEVER be applied for d-block electron configurations of transition metal compounds!

V 5-coordinate species and 7-coordinate species are special as they have two different bond lengths

1. The two bond types are axial and equatorial (Ref. Figure 24)

   1. for 5-coordinate species, axial positions possess more 90° repulsions than equatorial positions and therefore electronegative elements are placed in axial positions while lone pairs are placed in equatorial positions to minimize repulsions.

   2. If all substituents are the same, axial bonds are longer than equatorial bonds due to larger repulsion.

   3. Conversely for 7-coordinate species, equatorial positions possess more strained (72°) repulsions than axial positions, and therefore placements are reversed.

3.4.2 LCP Model

VI The Ligand Close Packing model utilizes distances between substituents in molecules as a guide to molecular shapes.

1. distances between two close substituents (that are not directly bonded to each other) are relatively constant even if bond angles and lengths change (based on experimental data)

2. unlike the VSEPR model that focuses on the central atom, the LCP model focuses on substituents.

   1. Therefore, the LCP model is used in conjunction with the VSEPR model.

3. the two species $NF_{4}^{+}$ and $NF_{3}$ is shown in Figure 25

VII Final word on VSEPR and LCP models:

1. A single bond to an electropositive substituent has electron density closer to the center of the molecule than a single bond to an electronegative substituent.

2. If electron density is further away from the center, there are less repulsions and able to adopt geometries with smaller angles between substituents. The opposite is also true.

3. Although the VSEPR model illustrated the reasoning behind geometries of many species, it did not explain the nature of bonding.