Ligand field theory and CuCl2 colors

  1. Copper (ii) chloride is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate.
    According to ligand field theory, water is a stronger field ligand than chloride. As a result, the dihydrate form should have a larger d orbital splitting than the anhydrous form. Thus, the color (complementary to the wavelength absorbed) of the dihydrate form should have a longer wavelength than the anhydrous form's. But why is the opposite observed here?
    If my reasoning is not correct, then please tell me the reason for the color change above.
    Thank you very much.
  2. jcsd
  3. Borek

    Staff: Mentor

    Unless I am missing something, you got it wrong. Larger d orbital splitting means higher energy difference so a shorter wavelength.
  4. Well, the color we see is complementary to the color absorbed.
  5. DrDu

    DrDu 4,639
    Science Advisor

    There are a zillion of possible explanations.
    There can be charge-transfer transitions invoved which have a much higher oscillator strength than the weak d-d transitions.
    Also in the hydrated chloride there are two water molecules along the axis and four chlorines in the equatorial plane, so this complex doesn't have a symmetry octahedral symmetry.
  6. Borek

    Staff: Mentor

    That's what I missed [​IMG]
  7. These are two answers I found on the net
    While both answers seem wrong to me, one mentioned Advanced Inorganic Chemistry 6th ed (1999). p 868-869 (F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann). If anyone has access to that book, please tell me what it really said.
    PS: I only have access to the third edition of that book, so I don't know the corresponding pages.
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