Valence layer electron pair mutual exclusion theory (1)

In 1940, NVSidgwick and others proposed the valence layer electron pair mutual exclusion theory (VSEPR), which was very successful in explaining the geometric configuration of ABn-type molecules or ions formed by main group elements
.

1.
Theoretical points

The geometric configuration of ABn-type molecules or ions depends on the repulsion of the electron pairs in the valence layer of the center A, and the configuration of the molecules always takes the form of the balance of the repulsive forces of electrons
.

1) The total number of electrons and the number of electron pairs in the central valence layer

The total number of electrons in the central atomic valence layer is equal to the number of valence electrons in the center A plus the number of electrons provided by the ligand during the bonding process.
For example, the total number of electrons in the ₄ valence layer of CH is 8, where C provides 4 valence electrons, and each H provides 1 Valence electrons
.

The oxygen group element provides 6 valence electrons when used as the center, 0 when used as terminal ligands, and 1 when used as non-terminal ligands
.
For example, the total number of electrons in the central valence layer of H ₂ O is 8, of which O provides 6 valence electrons as the center; the total number of electrons in the central valence layer of CO ₂ is 4, and O is used as a ligand to provide 0 valence electrons; the CH ₃ OH central valence layer The total number of electrons is 8, of which OH serves as a ligand to provide 1 valence electron

When the halogen atom is used as the center, it provides 7 valence electrons, and when it is used as the ligand, it provides 1 electron
.
For example, the total number of electrons in the central valence layer of IC1 ₃ is 10, where the I atom provides 7 valence electrons as the center, and each Cl provides 1 electron when Cl is used as a ligand

When dealing with the ion system, the number of electrons corresponding to the ion charge must be added or subtracted
.
For example, the total number of electrons in the central valence layer of SO ₄ ^2- is 8, of which negative charges provide 2 valence electrons; the total number of electrons in the central valence layer of NH ₄ ⁺ is 8.

Divide the total number of electrons in the valence layer by 2 to get the number of electrons.
When the total is an odd number, it is calculated as a carry
.
For example, if the total number of electrons in the central valence layer of NO ₂ is 5, the number of electron pairs is 3

2) The relationship between the electron pair and the space configuration of the electron pair

Electron pairs repel each other.
According to the number of electron pairs, the orientations of electron pairs to achieve repulsive balance in space are linear, regular triangle, regular tetrahedron, triangular double cone, regular octahedron (Table 6-4)
.

Table 6-4 The relationship between the number of electron pairs in the central valence layer and the configuration of electron pairs

3) The relationship between the geometric configuration of the molecule and the configuration of the electron pair

If the number of ligands is the same as the number of electron pairs, and all electron pairs are bonded electron pairs, then the molecular configuration and the electron pair configuration are the same
.
If the number of ligands is less than the number of electron pairs, part of the electron pairs are bonded electron pairs, and the remaining electron pairs are lone electron pairs.
The position of the lone electron pair is determined, and the molecular configuration can be determined

When there are 5 pairs of electrons in the central valence layer, the configuration of the electron pair is triangular double cone.
If there is 1 lone electron pair, the lone electron pair is placed in the axial position of the triangular double cone or on the plane triangle (the waist of the triangular double cone) s position?

In the triangular biconical configuration, the smallest angle between the electron pair is 90°, and the electron repulsion at the angle of 90° determines the position of the lone electron pair
.
The electron pair configuration always adopts the equilibrium position where the electron pair repulsion is the smallest.

Solitary electron pair-solitary electron pair>solitary electron pair-bonded electron pair>bonded electron pair-bonded electron pair

The configuration of the electron pair is a triangular double cone.
If there is a lone electron pair, there are two possibilities for the arrangement of the lone electron pair as shown in Figure 6-12 (a) and (b), and neither of the two structures is at an angle of 90°.
The electron pair-solitary electron pair repels, and the repulsive force is determined by the lone electron pair-bonded electron pair
.
In Fig.

The configuration of the electron pair is a triangular double cone.
If there are 2 lone electron pairs, there are three possibilities for the arrangement of the lone electron pairs in Figure 6-12 (c), (d) and (e)
.
In Fig.

Figure 6-12 A lone electron pair in a triangular biconical configuration

In the same way, when the configuration of the electron pair is a triangular double cone, the lone electron pair is always located at the position of the plane triangle (the waist of the triangular double cone) to minimize the repulsive force between the electron pairs
.

Table 6-5 The relationship between the geometric configuration of the molecule and the configuration of the electron pair