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    Delocalized π bond

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    1.
    Delocalized π bond

    1) Delocalization II 4 3 keys

    In the O 3 molecule, the central atom adopts sp 2 hybridization, as shown in Figure 6-29(a)
    .
    The sp 2 hybrid orbital of the central O atom and the 2p orbitals of the two ligand O each form a σ bond.


    The molecular structure is V-shaped, and the unhybridized 2p z orbital of the central atom has 2 electrons, as shown in Figure 6-29 ( b) as shown


    The 2p z orbitals of the two ligands O have single electrons, the bonds are not saturated, and both have the ability to form bonds
    .
    However, the 2p z orbitals of the two ligands O are far apart and cannot overlap to form a bond


    .


    Figure 6-29 O 3 molecular center hybridization, molecular structure and delocalized π bond

    The delocalized π bond in the O 3 molecule can be understood as a π bond formed by overlapping the p orbitals of two ligand O with the p orbital of the central atom
    .
    The electrons of the delocalized π bond belong to the entire molecule, and this bond is also called a large π bond


    .


    SO 2 , HNO 3 , and NO 2 - all have Delocalization Ⅱ 43 bonds, as shown in Figure 6-30
    .
    SO 2 and NO 2 - are isoelectronic bodies of O 3 , so the three have similar structures and bonds


    .

    FIG SO 6-30 2 , HN03 .
    3
    , and NO 2 - T formed delocalized Ⅱ .
    4
    .
    3
    bonds

    The molecular orbital energy level diagram of Ⅱ 4 3 in O 3 molecule is shown in Fig.
    6-31(a).
    The combination of 3 pz orbitals forms 3 π orbitals, 1 bonding orbital, 1 antibonding orbital, and 1 non-bonding orbital.
    Key track .
    .
    4
    .
    3
    key on the key molecule is a contribution level, the contribution level of each 1/2 bond OO bond, i.
    e.
    , O .
    3
    molecule OO bond in the order of 1.
    5 .
    It can be seen that O 3 molecules are not as stable as O 2 (the bond level is 2) .
    O 3 molecule has no single electron, so O 3 is a diamagnetic molecule .




    Fig.
    6-31 Molecular orbital energy level diagram and electron arrangement forming bonds of Ⅱ 4 3 and Ⅱ 6 4

    The O .
    3
    is formed Ⅱ molecule .
    4
    .
    3
    process can be summarized from the domain π bond-forming conditions are as follows:

    (1) Several atoms are as coplanar as possible (the orbits can effectively overlap "side by side")
    .

    (2) There are orbitals perpendicular to the molecular plane (each orbital participates in the formation of delocalized π bonds)
    .

    (3) The total number of electrons in the orbital is less than twice the number of orbitals (the contribution of delocalized π bonds to the bond level is not 0)
    .

    2) Delocalization Ⅱ 6 4 key

    In the SO 3 molecule, the central atom S excites a 3s orbital electron to enter the 3d orbital to ensure that there are 3 single electrons in the sp 2 hybrid orbital of S and 2 electrons in the 3pz orbital of the unhybridized central atom; the central S atom The sp 2 hybrid orbital of O and the 2p orbital of the 3 ligand O each form a σ bond, and the molecular structure is an equilateral triangle, as shown in Figure 6-32
    .

    Figure 6-32 The chemical and molecular structure of the central atom of SO 3 molecule

    The 2p z orbitals of the three ligand O in the SO 3 molecule have single electrons, which are parallel to the 3pz orbitals of the central S atom and have the same symmetry.
    They can be overlapped side by side to form a delocalized π bond; at the same time, the S atom is excited The electron to the 3d orbital enters the delocalized π bond, SO 3 forms a four-center six-electron delocalized π bond II 6 4 , as shown in Figure 6-33(a) .


    The delocalized π bond in the SO 3 molecule can be understood as the p orbitals of the three ligand O overlap each other with the help of the p orbital of the central S atom to form a π bond .


    Figure 6-35 Delocalized Ⅱ 6 4 bond in SO 3 and BCl 3 molecules

    The energy level diagram of the molecular orbitals in the SO 3 molecule is shown in Figure 6-31(b).
    The combination of 4 pz orbitals forms 4 π orbitals, 1 bonded orbital, 1 antibond orbital, and 2 non-bonded orbitals


    .


    And SO .
    3
    is similar to, the BCl .
    3
    , NO .
    3
    -, CO.
    'S .
    3
    2 - and the like are delocalized Ⅱ .
    6
    .
    4
    key
    .
    Moreover, SO 3 and BCl 3 , NO 3 - and CO 3 2- are mutually isoelectronic bodies


    .


    In the BCl 3 molecule, B adopts sp 2 hybridization (Figure 6-17).
    The three hybrid orbitals are paired with the 3p orbitals of Cl to form a σ bond.
    The remaining 2p z orbitals that are not hybridized have electron pairs with Cl.
    The 3p z orbitals overlap, so there is a delocalized Ⅱ64 bond in the BCl 3 molecule [Figure 6-33(b)]
    .
    The formation of the Ⅱ64 bond increases the bond level of BCl 3 and the B-Cl bond in the molecule, alleviates the electron deficiency of B, and improves the stability of the BCl 3 molecule


    .


     

     

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