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    Composition and classification of coordination compounds

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    1.
    The concept of coordination compounds

    A complex molecule or ion formed by the combination of a central atom (or ion) and several ligand molecules (or ions) through coordination bonds is usually called a coordination unit or a coordination unit
    .
    Compounds containing coordination units are called coordination compounds, or complexes for short

    .

    The coordination unit can be a cation, such as [Co-(NH 3 ) 6 ] 3+ and [Cu(NH 3 ) 4 ] 2+ ; it can be an anion, such as [Cr(CN) 6 ] 3- and [Co(SCN ) 4 ] 2- ; It can also be a neutral molecule, such as [Ni(CO) 4 ] and [Cu(NH 2 CH 2 COO) 2 ]
    .

    The coordination unit combines with the ion of the opposite charge to form a complex, such as [Co(NH 3 ) 6 ]Cl 3 , K 3 [Cr(CN) 6 ], [Co(NH 3 ) 6 ][Cr(CN) 6 ], and neutral coordination units are complexes, such as [Ni(CO) 4 ] and [Cu(NH 2 CH 2 COO) 2 ]
    .

    According to the definition of coordination compound, SO 4 2- is also a coordination unit, and K 2 SO 4 should also be regarded as a complex, but it is customary not to regard it as a complex
    .
    For another example, Mohr's salt (NH
    4 ) 2 SO 4 ·FeSO 4 ·6H 2 O contains [Fe(H 2 O) 6 ] 2+ units, but people call Mohr's salt a double salt, instead of considering it.
    Into a complex

    .
    The hydrated ions formed by water molecules as ligands are often not regarded as complex ions

    .

    2.
    The composition of the coordination compound

    Coordination compounds generally consist of two parts: the inner boundary and the outer boundary
    .
    The coordination unit is the inner boundary, and the ion with the opposite sign of the inner boundary is the outside.
    For example, in the complex [Co(NH
    3 ) 6 ]Cl 3 , the inner boundary is [Co(NH 3 ) 6 ] 3+ , The outside is CI - ; in the complex K3[Co(CN) 6 ], the inside is [Co(CN) 6 ] 3- and the outside is K +
    .
    The neutral coordination unit as a complex has no outside world, such as [Ni(CO)
    4 ]; in the complex [Co(NH 3 ) 6 ] [Cr(CN) 6 ], it can be considered as [Co(NH 3 ) 6 ] 3+ and [Cr(CN) 6 ] 3- are both inner bounds, or the two are considered to be both inside and outside
    .

    In an aqueous solution, the inner and outer parts of the coordination compound are completely dissociated, and the coordination unit that is the inner boundary is relatively stable, and the degree of dissociation is relatively small.
    In the aqueous solution, there is coordination between the coordination unit and the center and the ligand.
    -Dissociation equilibrium

    .

    The inner boundary of the coordination compound is composed of the center and the ligand
    .
    The center is also called the forming body of the coordination compound, and it is mostly metal

    .
    Center can be positive ions (mostly metal ion), such as [FeF
    .
    6
    ] 3+ in Fe3 + and [of Co (NH2 .
    3
    ) .
    6
    ] 3+ is of Co 3+ may be atoms, such as [of Ni (CO.
    'S)
    .
    4
    ] Ni in [Fe(CO) 5 ] and Fe in [Fe(CO) 5 ]; the oxidation number of the center can also be negative, such as Co in Na[Co(CO) 4 ]
    .

    The ligand can be a molecule, such as NH2 .
    3
    , H 2 O, CO.
    'S, N
    2 , an organic amine and the like; may be an anion such as F.
    - , the I - , OH - , the CN - , the SCN - , C 2 O .
    4
    2- , CH .
    3
    -COO- the like
    .

    3.
    Coordination atom and coordination number

    Coordinating atoms are atoms in the ligand that provide electron pairs to directly form a coordination bond with the center, such as N in NH 3 , O in H 2 O, C in CO, O and N in NH 2 CH 2 COO, etc.

    .

    The coordination number is the number of coordination atoms around the central atom that directly bond with the center
    .
    Be careful not to confuse the coordination number with the number of ligands

    .
    For example, the coordination number in [Co(NH3)6]Cl3 is equal to the number of ligands; the coordination number in [Cu(NH
    2 CH 2 COO) 2 ] is not equal to the number of ligands
    .
    Most of the coordination numbers are even numbers (2, 4, 6, 8), of which 4 and 6 are the most; coordination units with odd numbers (3, 5, 7) are fewer

    .

    The number of coordination numbers is related to the charge and radius of the center and the charge and radius of the ligand
    .
    The high charge and large radius of the central ion are conducive to the formation of coordination units with high coordination numbers

    .
    The center with an oxidation number of +1 is easy to form a two-coordinated coordination unit, such as [Ag(CN)
    2 ] - , [Ag(S 2 O 3 ) 2 ] -, etc.
    ; the center with an oxidation number of +2 is easy to form four Coordination or six coordination coordination units, such as [Cu(NH
    2 ) 4 ] 2+ , [Zn(NH 3 ) 4 ] 2+ , [Co(NH 3 ) 6 ] 2+ , [Fe(CN) 6 ] 4- etc.
    ; the center with oxidation number +3 can easily form six-coordinated coordination units, such as [Co(NH
    3 ) 6 ] 3+ , [Fe(CN 6 )] 3- , [Cr(CN)6 ] 3- and the like
    .

    The larger the radius of the ligand, the less the ligand that can be accommodated around the center, which is not conducive to the formation of coordination units with high coordination numbers; the ligand has more negative charges, although the attraction to the center is increased, the ligand The repulsive force between the increases, and the overall result is a decrease in the coordination number
    .
    For example, Fe3+ and F- with a small radius can form six-coordinated [FeF
    6 ] 3- , but with a larger radius of CI -, it mainly forms four-coordinated [FeC1 4 ] - ; Fe 3+ and C 2 O 4 2- Forms six-coordinated [Fe(C 2 O 4 ) 3 ] 3- , but can only form [Fe(PO 4 ) 2 ] 3- with PO 4 2- .

    The number of coordination numbers is also related to factors such as temperature and ligand concentration
    .
    The temperature rises and the thermal vibration intensifies, which reduces the coordination number; the increase in the concentration of the ligand is conducive to the formation of high coordination number coordination units

    .
    For example, as the concentration of CN
    - changes, it can form a coordination unit with a coordination number of 1 to 4 with Cu +
    .

    4.
    Simple complexes and integrants

    The complex formed by the center and the single-base (dentate) ligand is called a simple complex
    .
    Single-base ligand refers to a ligand with only one coordinating atom

    .

    The complex formed by the center and the multi-base ligand is called an integrant because it forms a stable five- or six-membered ring, such as the complex ion formed by Cu 2+ and ethylenediamine (en) [Cu(en) 2 ]2+ [Figure 9-1(a)]
    .
    Multi-base ligands refer to ligands with two or more coordinating atoms, such as
    ethylenediamine and ethylenediaminetetraacetic acid (EDTA)
    .

    The neutral coordination unit formed by the negative ion multi-base ligand and the positive ion center is called the internal salt.
    For example, Cu
    2+ forms an internal salt with aminoacetate [Figure 9-1(b)]
    .

    Figure 9-1 The structure of [Cu(en) 2 ] 2+ and [Cu(NH 2 CH 2 COO) 2 ]

     

     

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