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    Isomerism of coordination compounds

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    The phenomenon in which coordination compounds have the same composition but different structures is called the isomerism of coordination compounds and is divided into two categories: structural isomerism (or structural isomerism) and stereoisomerism (or steric isomerism)
    .

    The composition of the coordination compound is the same, but the bonding relationship between the ligand and the center is different, and structural isomerism will occur; the composition of the coordination unit is the same, and the bonding relationship between the ligand and the center is also the same, but there are ligands around the center.
    The relative position between the different or the different arrangement order in space, then stereoisomerism occurs
    .

    1.
    Structural heterogeneity

    Structural isomerism includes dissociative isomerism, coordination isomerism and bonding isomerism
    .

    1) Dissociation

    In water, the coordination compound is completely dissociated between the inside and the outside
    .


    The structural isomerism between the coordination compound and the original coordination compound obtained by exchanging components between internal and external components is called dissociative isomerism


    Two coordination compounds that are dissociative and isomerized to each other have different types of dissociated ions
    .


    For example, [CoBr(NH 3 ) 5 ]SO 4 (purple) and [CoSO 4 (NH 3 ) 5 ]Br (red), the former can precipitate Ba 2+ , and the latter can precipitate Ag +


    H 2 O is often in the inner boundary as a ligand, while crystal water exists in the outside
    .


    The dissociation and isomerism caused by the difference of H 2 O molecules in the inner or outer boundaries are also called hydration isomers, such as [Cr(H 2 O) 6 ]Cl 3 (purple) and [CrCl(H 2 O) 5 ]Cl 2 ·H 2 O (light green) is mutually hydrated isomers


    2) Coordination isomerism

    In coordination compounds composed of coordination anions and coordination cations, both anions and cations are coordination units
    .


    The ligands are exchanged between the coordination units to obtain coordination isomers, such as [Co(NH 3 ) 6 ][Cr(CN) 6 ] and [Cr(NH 3 ) 6 ][Co(CN) 6 ] mutually Coordination isomers


    3) Bonding isomerism

    There are two coordinating atoms in the ligand, but the two coordinating atoms are not coordinated at the same time.
    Such a ligand is called a dual ligand
    .


    For example, NO 2- may be oxygen atoms, to form nitrite (-ONO, - ) coordination compound as a ligand; with the nitrogen atom may be coordinated, in order to form a nitro (-NO2 2- ) is equipped Coordination compound of the body


    The two ligands lead to coordination compounds with bonding isomers, such as [Co(NO 2 )(NH 3 ) 5 ]Cl 2 and [Co(ONO)(NH 3 ) 5 ]Cl 2
    .

    2.
    Stereoisomerism

    Stereoisomerism includes geometric isomerism (also called cis-trans isomerism) and optical isomerism (or enantiomerism)
    .

    1) Geometrical heterogeneity

    The coordination unit of a regular tetrahedral structure with a coordination number of 4, regardless of whether the four ligands are the same, will not have geometric isomers
    .

    For the coordination unit with a planar quadrilateral structure with a coordination number of 4, Mab 3 has no geometric isomers, Ma 2 b 2 has 2 geometric isomers, and Mabc 2 has 2 geometric isomers
    .


    For example, the [PtCl 2 (NH 3 ) 2 ] of the planar quadrilateral structure has cis (cis-[PtCl 2 (NH 3 ) 2 ]) and trans (trans-[PtCl 2 (NH 3 ) 2 ]) geometry Isomer (Figure 9-2)


    Figure 9-2 The cis and trans isomers of [PtCl 2 (NH 3 ) 2 ]

    The geometric isomerism of octahedral coordination compounds with a coordination number of 6 is more complicated.
    Ma 2 b 4 , Ma 3 b 3 and Mabc 4 each have 2 geometric isomers, and Mab 2 c 2 has 3 geometric isomers.
    , Ma 2 b 2 c 2 has 5 geometric isomers (Figure 9-3)
    .



    Figure 9-3 Geometric isomers of octahedral coordination compounds

    In short, the more the number of ligands, the more types of ligands, and the more complex geometric isomerism
    .

    2) Optical isomerism

    Two coordination units that are mirror images of each other are called optical isomers.


    The positions of the ligands or coordinating atoms are the same, but due to the different orientations of the ligands in space, the two are isomers that cannot overlap


    To determine whether there are optical isomers in a certain geometric configuration of the coordination compound (or ligand ion), it is usually based on whether there is a symmetry plane or center of symmetry in the geometric configuration of the coordination unit.
    If there is, there is no optical isomer.
    Structure; if not, there are optical isomers
    .

    Among the six-coordinate octahedral coordination unit, the cis Mazbaca has optical isomers, cis-[Cr(en) 2 Cl 2 ] + (Figure 9-4), [Co(en) 3 ] 3+ , [Fe(C 2 O 4 )3] 3- There are also optical isomers
    .

    Figure 9-4 Schematic diagram of octahedral optical isomers

    In theory, four-coordinated tetrahedral coordination compounds have optical isomers if the four ligands are different (Mabcd), but it is difficult to obtain in the laboratory
    .

     

     

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