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Topic: Why is Iron (III) Chloride yellow? (crystal field theory)  (Read 1169 times)

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Offline Traumatic Acid

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Why is Iron (III) Chloride yellow? (crystal field theory)
« on: October 18, 2018, 11:49:57 PM »

Cl- is a weak field ligand, no? So why would Iron(111)chloride appear yellow? As a weak field ligand it would take a photon of lower energy to equate to Δo, so why is violet light being absorbed?

Offline Flatbutterfly

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Re: Why is Iron (III) Chloride yellow? (crystal field theory)
« Reply #1 on: October 22, 2018, 05:47:34 PM »
“there are also tetrahedral FeCl4]^-, trigonal bipyramidal [FeCl5]^2-, octahedral [FeCl6]^3-…..” [1]
[FeCl4]^- is also pale yellow that indicates that the color of these cmplx ions is not due to d-d transitions.
Fe(III) is [Ar]3d^5 and the d orbital splitting diagram (H2O a weak field ligand) is
t2g (↑)(↑)(↑) →Δoct →  (↑)(↑)eg   (high spin)
As can be seen the d-d transitions are spin forbidden and for our purposes have negligible intensity.  The color can be confidently assigned to a ligand-to-metal charge transfer (LMCT) band that involves  promotion of an electron in a filled Cl^- p AO to an Fe^3+ 3d AO (more correctly an MO).  Such bands are strongly allowed but usually occur at high energies (UV).  The yellow color is the result of the LMCT band tailing into the violet region of the visible spectrum; yellow is the complementary color to violet.  [1]
[1] F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann,  Advanced Inorganic Chemistry 6th ed (1999) p 788; 790.

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