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Homework Help: Chemistry color of complex ions

  1. Apr 26, 2007 #1
    1. The problem statement, all variables and given/known data
    Among the complex ions formed by [tex]Co^{3+}[/tex] match each ion to its color.

    1 [tex][Co(CN)_{6}]^{3-}[/tex]
    2 [tex][Co(NH_{3})_{6}]^{3+}[/tex]
    3 [tex][CoF_{6}]^{3-}[/tex]
    4 [tex][Co(H_{2}O)_{6}]^{3+}[/tex]

    A Blue
    B Green
    C Pale Yellow
    D yellow-orange

    3. The attempt at a solution

    I found the ligands for each mode.
    1 CN-
    2. NH3
    3. F-
    4. H2O

    I looked up the values and found that the wavelengths from shortest to longest is CN-, NH3, H2O then F-

    Therefore, CN is blue NH3 is green H2o is pale yellow and then F- is orange yellow. This is incorrect and I don't know why. Thanks for your help.
  2. jcsd
  3. Apr 26, 2007 #2
    Im not sure if this will help you, but complexes dont color because of the ligand. Its the number of unpaired electrons present in the central atom. That might help you out, although I dont know how you would go about finding the wavelengths.
  4. Apr 26, 2007 #3


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    Not so....
    Ligands do cause the ion to absorb different wavelengths. The theory you need to refer to is crystal field theory. If you are familiar with the strong or weak field ligands you should be able to answer this. Have you ever discussed "10 Dq" in your lectures?

    Remember, something that absorbs in the red will appear green and something that absorbs in the blue will appear yellow.

    This from Wiki:

    λ absorbed versus colour observed
    400nm Violet absorbed, Green-yellow observed (λ 560nm)
    450nm Blue absorbed, Yellow observed (λ 600nm)
    490nm Blue-green absorbed, Red observed (λ 620nm)
    570nm Yellow-green absorbed, Violet observed (λ 410nm)
    580nm Yellow absorbed, Dark blue observed (λ 430nm)
    600nm Orange absorbed, Blue observed (λ 450nm)
    650nm Red absorbed, Green observed (λ 520nm)

    These are arranged from highest energy absorbed to lowest.
    Last edited: Apr 26, 2007
  5. Apr 27, 2007 #4

    I know a bit about the crystal field theory/strong ligands/weak ligands, but what is 10dq? Maybe I know about it but don't know the terminology?
  6. Apr 27, 2007 #5


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    10Dq is very like the delta value that is used in Crystal Field Theory. Its magnitude determines if the crystal field is strong or weak and tells you something about the absorbance spectrum of the metal complex.

    See here for details:

    http://www.chem.tamu.edu/rgroup/dunbar/Chem634/notes(203_222).doc [Broken]
    Last edited by a moderator: May 2, 2017
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