[ptryon]

Hi,

According to crystal field theory, the potential energy of d-orbitals of a transition metal are split by the electric fields of approaching ligands and this is widely accepted as an explanation of why transition metal compounds are coloured.

The normal explanation for colour given in A-level textbooks is that the energy gap between the orbitals is equal to the energy of a particular frequency of visible light. Therefore if white light is shone onto a sample a particular frequency of light will be absorbed (causing an electron to excite between the d-orbitals) and the remaining frequencies will be reflected or transmitted and observed as a particular colour. This explanation seems to me to be wrong on at least two counts:

(1) If an electron is excited when light is absorbed, the electron will de-excite emitting exactly the same frequency of radiation that was initially absorbed. Therefore, overall no light is absorbed, and the sample should appear white.

(2) If the ligands cause the energy levels to split and we only have one type of ligand (and therefore one complex) present, their will only be one specific frequency of light absorbed (compared to the full range of possible frequencies in the visible spectrum). As a consequence the remaning frequencies will still combine to appear white.

I understand that the real explanation is more complicated and I am fine with the fact that we have to simplify things for learners. But I am not sure whether it is helpful to give explanations that are simplified to the point that they don't actually explain the phenominon in question. Does anyone know a more detailed explanation for the origin of colour that avoids these problems?
 

[mjc123]

(1) a. The emitted light is emitted in all directions, so in the direction of the beam the intensity is reduced at the frequency absorbed.
b. Not all the light absorbed is re-emitted at the same frequency. A lot of the energy is converted to vibrational, rotational and translational energy. That's why things warm up when they absorb light.

(2) You never get purely monochromatic absorption. There are always factors that cause broadening of the lines. In the case of transition metal complexes in solution, the main source of broadening is the excitation of vibrational states along with the electronic transition (giving what are called vibronic transitions). In the gas phase individual vibronic transitions might be resolvable, but in solution they are broadened and overlap to form one broad absorption band. Hence a significant range of the spectrum is absorbed.

[Irlanur]

Sometimes the most important transition concerning colour is the transition between 2 d-orbitals. usually, it's not. e.g. MnO(4-)