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Structural Isomers

    Isomers are compounds that have the same molecular formulas but differ slightly in structure or composition. Structural isomers only differ in structure or bond type. There are three types of structural isomers: ionization, coordination and linkage.

    1. 1. Introduction
      1. 1.1. Ionization Isomerism
      2. 1.2. Coordination Isomerism
      3. 1.3. Linkage Isomerism
    2. 2. References
    3. 3. Problems
    4. 4. Answers

    Introduction

    Structural isomers, as their name implies, differ in their structure or bond type. This is different from stereoisomers, which differ in where the ligands are attached but still have the same kindsof ligands attached. The notable difference between structural isomers and stereoisomers is structural isomers have basically the same chemical formulas but with different bonding arrangements, while stereoisomers have identical chemical formulas. The different chemical formulas in structural isomers are caused either by a difference in what ligands are bonded to the central atoms or how the individual ligands are bonded to the central atoms. When determining a structural isomer, you look at:

    1. the ligands that are bonded to the central metal, and
    2. which atom of the ligands attach to the central metal. 

    Below is a quick look at the different types of structural isomers. The highlighted ions are the ions that switch or change somehow to make the type of structural isomer it is.

    Structural Isomer 

    Example 

    Ionization 

    [CoBr(H2O)5]+Cl-  and [CoCl(H2O)5]+Br- 

    Coordination 

    [Zn(NH3)4]+[CuCl4]-2 and [Cu(NH3)4]+[ ZnCl4]-2 

    Linkage 

    [Co(NO2)6]-3 and [Co(ONO)6]-3 

    Ionization Isomerism

    Ionization isomers are identical except for a ligand has exchanging places with an anion or neutral molecule that was originally outside the coordination complex. The central ion and the other ligands are identical. For example, an octahedral isomer will have five ligands that are identical, but the sixth will differ. The non-matching ligand in one compound will be outside of the coordination sphere of the other compound. Because the anion or molecule outside the coordination sphere is different, the chemical properties of these isomers is different. A hydrate isomer is a specific kind of ionization isomer where a water molecule is one of the molecules that exchanges places.

    Example  

     img1_ionization.jpg

    We have pentaaquabromocobaltate(II)chloride which changes to pentaaquachlorocobaltate(II)bromide.

    Coordination Isomerism

    Coordination isomers occur with coordination compounds that are composed of both a cation complex and an anion complex, meaning there are two complex compounds bound together, one with a negative charge and the other with a positive charge. In coordination isomers, the anion and cation complexes of a coordination compound exchange one or more ligands.

    Example  

     img2_coordination.jpg

    Linkage Isomerism

    The linkage isomers of a coordination complex have the same ligands and central atom, and the ligands are attached in the same locations. The only difference is what atoms the molecular ligands use to attach to the central ion.  The ligand(s) must have more than one donor atom, but bind to ion in only one place. For example, the (NO2-) ion is a ligand can bind to the central atom through the nitrogen or the oxygen atom, but cannot bind to the central atom with both oxygen and nitrogen at once, in which case it would be called a polydentate. The formula of the complex is unchanged, but the properties of the complex may differ.  The names used to specify the changed ligands are changed as well.  For example, the (NO2-) ion is called nitro when it binds with the N atom and is called nitrito when it binds with the O atom.

    Example  

     img3_linkage.jpg

     

    References

    1. Petrucci, Harwood, Herring, and Madura. General Chemistry: Principles and Modern Applications: Ninth Edition. New Jersey: Pearson, 2007.
    2. Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight: Fourth Edition. W. H. Freeman and Company, 2008. Print.

    ProblemsEdit section

    1. Write the Coordination Isomer for:   [Co(NH3)6][Cr(CN)6]
    2. Write the corresponding linkage isomer as well as names of the two complexes for:   [CoCl(NO2)(NH3)4Cl]
    3. What is the coordination isomer of:   [Cr(NH3)6][Fe(CN)6]
    4. Write the Ionization isomer for:   [CoBr(NH3)5]SO4
    5. Explain a polydentate ligand.

    Answers

    1. [Cr(NH3)6][Co(CN)6]
    2. [CoCl(ONO)(NH3)4Cl]
    3. [Fe(NH3)6][Cr(CN)6]
    4. [CoSO4(NH3)5]Br
    5. A polydentate ligand is a ligand that can bind to the central atom of a complex compound at many places at one time.

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