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.
- At low temperatures, CuCl42- is square planar or distorted square planar.
- At high temperatures, CuCl42- is tetrahedral.
- Δt is ~0.44 Δo, but Δsp is ~1.4Δo, therefore Δt < Δsp.
- 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.
- 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?