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5.5.4: Structural Isomers - Linkage Isomerism in Transition Metal Complexes

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    Linkage isomerism occurs with ambidentate ligands that are capable of coordinating in more than one way. The best known cases involve the monodentate ligands: \(SCN^- / NCS^-\) and \(NO_2^- / ONO^-\). The only difference is what atoms the molecular ligands bind 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 rather than an ambidentate ligand.

    Linkage.jpg

    Figure \(\PageIndex{1}\): Linkage Isomerism in the \(NO_2\) ligand. This occurs when a particular ligand is capable of coordinating to a metal in two different and distinct ways.

    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 \(\PageIndex{1}\): Nitro- vs. Nitrito- Linkage Isomers

    The cationic cobalt complex [Co(NH3)5(NO2)]Cl2 exists in two separable linkage isomers of the complex ion: [Co(NH3)5(NO2)]2+.

    300px-LinkageIsomers.png
    (left) The nitro isomer (Co-NO2) and (right) the nitrito isomer (Co-ONO)

    When donation is from nitrogen to a metal center, the complex is known as a nitro- complex and when donation is from one oxygen to a metal center, the complex is known as a nitrito- complex. 

    • \([Co(ONO)(NH_3)_5]Cl\): the nitrito isomer -O attached
    • \([Co(NO_2)(NH_3)_5]Cl\): the nitro isomer - N attached.

    The formula of the complex is unchanged, but the properties of the complex may differ.

    Another example of an ambidentate ligans is thiocyanate, SCN−, which can attach at either the sulfur atom or the nitrogen atom. Such compounds give rise to linkage isomerism. Polyfunctional ligands can bond to a metal center through different ligand atoms to form various isomers. Other ligands that give rise to linkage isomers include selenocyanate, SeCN− – isoselenocyanate, NCSe− and sulfite, SO32−.

    Exercise \(\PageIndex{1}\)

    Sketch the linkage isomers of [FeCl5(SCN)]3-.

    Answer

    In the first isomer, the ligand bonds to the metal through the nitrogen atom. In the second isomer, the ligand bonds to the metal through the sulfur atom. It's easier to see it:

    Linkage2.jpg

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    5.5.4: Structural Isomers - Linkage Isomerism in Transition Metal Complexes is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.