# Comparing intensity of colour in coordination compounds

I had given an examination recently in which the question asked was:

Write the decreasing order for the intensity of color in $$\ce{[CoCl4]^{2-}}$$, $$\ce{[Co(CN)4]^2-}$$, $$\ce{[Co(H2O)6]^2+}$$

What my approach was to first find out the geometry of compounds after which I found the geometry as follows
$$\ce{[CoCl4]^{2-}}$$ : $$\mathrm{sp^3}$$,
$$\ce{[Co(CN)4]^2-}$$ : $$\mathrm{dsp^2}$$,
$$\ce{[Co(H2O)6]^2+}$$ : $$\mathrm{sp^3d^2}$$

Now, I approached the question with the thought that more the symmetry less is the intensity of the color of the compound, which gave me a conclusion about what I had learned earlier that intensity of color in $$\mathrm{sp^3 > sp^3d^2}$$. And as the symmetry of $$\mathrm{dsp^2}$$ is comparable to that of $$\mathrm{sp^3d^2}$$, in that the intensity of color would be more for strong field ligand.

This made me conclude to the answer $$\ce{[CoCl_{4}]^{2-} > [Co(CN)_{4}]^{2-} > [Co(H2O)_{6}]^{2+}}$$.

But I am really confused and not sure about this answer as maybe the intensity of the color is completely determined only by the nature of the ligand, or geometry is involved or not. I also have confusion about how should I compare the intensity of color in $$\mathrm{dsp^2}$$ and $$\mathrm{sp^3d^2}$$.

P.S.: If possible please answer the question with basics as I am a High School student and don't know much about Higher Chemistry.

• I don't think hybridization helps to predict intensity. Can you add what the textbook says? Jul 21 '21 at 14:22
• Unfortunately: Why is it wrong to use the concept of hybridization for transition metal complexes? There won't be a simple answer; there might be a retrofit of an explanation using simpler models, but that will fit the situation only. Jul 21 '21 at 21:06
• Bhanu, I never heard of that. Ask your teacher to give a reference. Jul 21 '21 at 21:56
• Qualitatively, asymmetric complex may allow breaking of the selection rules and its intensity may be higher. I do not recall seeing your teacher's explanation. Jul 21 '21 at 23:00
• Anyway, let us take it literally that they want you to predict intensity of transitions in complexes. "Intensity" in absorption spectroscopy has a different meaning altogether. It means how strong is the absorption of light (molar absorptivity). It is not a trivial task to predict just by this given information. It is a cruelty to ask this question from college students. The second problem with this question is that it is asking you to compare three tetrahedral or perhaps square planar complexes with an octahedral one. Jul 22 '21 at 23:25