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2.1.5: Isomers

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    200859

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    Sometimes, two compounds can have the same molecular formula but different structures. In transition metal complexes, that can happen if the same ligands are arranged around the metal in different ways.

    For example, the complex ion [CoCl2(NH3)4]+ contains two chlorine ligands that can be placed in two different ways around the cobalt. The cobalt in this complex has octahedral geometry. Every ligand in the complex is 90 degrees away from four other ligands, and 180 degrees away from a fifth. If there are two chloride ligands, then they might be found 90 degreed away from each other, or they might be found 180 degrees away from each other. The former case exists, and it is called the cis-isomer; the latter case also exists, and is called the trans-isomer (cis means same, as in same side, as opposed to trans, which means opposite).

    Left: cis complex ion, with two chlorines on the same side, 90 degrees apart. Right: trans complex ion, with two chlorines on opposite sides, 180 degrees apart.

    In this case, the two compounds have different physical properties, most readily seen in the two different colors of the complexes: one is purple and the other is green. The exact reasons for the colors are subtle; you're not expected to know why one is green and the other is purple at this stage.

    Exercise \(\PageIndex{1}\)

    Draw structures for the following compounds:

    1. trans-Fe(py)4Cl2
    2. trans-Mn(OH2)4F2
    3. cis-(bpy)V(acac)Cl2
    4. cis-Mo(CO)4(PMe3)2
     

    A group of three ligands can also adopt two different geometries in an octahedron. The three ligands can be found clustered together on one side of the complex. They occupy one face of the octahedron, in what is called the fac-geometry. Alternatively, they could lie all in a row. Sitting along a meridian of the octahedron, they adopt a mer-geometry.

    Fac and mer geometries of complex ions.

    Exercise \(\PageIndex{2}\)

    Draw structures of the following complexes:

    1. fac-W(CO)3(NCCH3)3
    2. mer-Mo(CO)3(PMe3)3
    3. fac-Mo(CO)3(PF3)3
    4. mer-Mo(CO)3(PCl3)3
    5. fac-Na[Mn(CO)3(CN)3]
    Answer a:

    Tungsten coordination complex with three carbon monoxides and three NCCH3 groups.

    Answer b:

    Molybdenum coordination complex with three trimethyl phosphate ligands and three carbon monoxide ligands.

    Answer c:

    Molybdenum coordination complex with three carbon monoxide ligands and three PF3 ligands.

    Answer d:

    Molybdenum coordination complex with three carbon monoxide ligands and three PCl3 ligands.

    Answer e:

    Manganese coordination complex with three carbon monoxide ligands and three cyanide ligands. Overall charge of -1. Sodium counterion.

    See a more in-depth discussion of coordination complexes in a later course.

     

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