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The colour of the compound changes from deep green/green-blue to yellow upon raising the temperature. I'm trying to rationalise this by using the d-orbital splitting, but I've got myself in a logic bind.

  1. At low temperatures, CuCl42- is square planar or distorted square planar.
  2. At high temperatures, CuCl42- is tetrahedral.
  3. Δt is ~0.44 Δo, but Δsp is ~1.4Δo, therefore Δt < Δsp.
  4. This therefore implies that at low temperatures (square planar geometry), the photon absorbed to give a colour is of higher energy, and so lower wavelength.
  5. Green corresponds to an absorption of 650 nm (in the red), whereas yellow corresponds to an absorption of 450 nm (violet absorption).

Therefore, the colours contradict my predictions. What is the cause of this?

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  • $\begingroup$ The colour you observe by eye is not the same as the wavelength that the species absorb: for example Chlorophyll is green but absorbs strongly in the red , 680-700 nm, ( as well as in near uv 400nm). (Also you mean 650 nm and 450 nm not wavenumbers) $\endgroup$ – porphyrin Feb 4 '18 at 15:23
  • $\begingroup$ Thanks, I've corrected the units. Also, for the colours - green corresponds to an absorption of 650 nm i.e. absorption of red light, and yellow to 450 nm (violet light absorbed). Therefore I believe that I have already taken your comment into account? $\endgroup$ – chemistrystudent Feb 4 '18 at 15:32
  • $\begingroup$ See the last part of this answer $\endgroup$ – Nilay Ghosh Feb 4 '18 at 16:24
  • $\begingroup$ Please correct me if I'm wrong, but in that answer, it is stated that the colour changes to yellow due to a transition from square planar to tetrahedral, and gives a MO diagram. However, I don't believe it explains why the wavelength of light absorbed falls during the transition? This corresponds to an increase in splitting which is typically not the case from square planar to tetrahedral - transitions to the b1g dx2-y2 should require more energy than transitions to the t2 dxy, dxz, dyz? $\endgroup$ – chemistrystudent Feb 4 '18 at 18:12

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