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Since ammonia is a stronger ligand than $\ce{H2O}$ shouldn't the colour of the new solution $\ce{[Cu(NH3)4(H2O)2]}$ be lighter as the energy gap will be higher and so light with higher frequency and energy (between blue and indigo) be absorbed which means that the transmitted light is the one between blue and green which is lighter? Why does the solution turn deep blue?

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Like the OP mentioned, the replacememt of a weak field ligand ($\ce{H2O}$) by a stronger one ($\ce{NH3}$) certainly increases the $\Delta_o$ of the $\ce{Cu(II)}$ complex.

A chemical sample appears colored when it absorbs a certain wavelength of light. What we see is the sum total of the remaining colors. The colour visible to the human eye would be the complement of the colour(i.e. wavelength) absorbed.

Color wheel

The increase in $\Delta_o$ leads to the absorption of a lower wavelength (or higher frequency) of light, as evident from: $$E=h\nu= \frac {hc}\lambda $$

color-freq correspondence

Note that a higher frequency would mean a shift towards the blue end.

Quantitatively speaking, the light blue colour of the initial hexaaqua complex was due to the absorption of the orange spectrum range of light.

Back to the color wheel, a shift towards the blue side (in the green direction, please) gives us a lighter shade of orange.

Why does the solution turn deep blue?

Observe the complement(the colour diametrically opposite) of orange-yellow is definitely a deeper shade of blue.

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  • $\begingroup$ So whatever is complement to the absorbed colours in the colour wheel will be the transmitted colors(i.e. ones observed)? $\endgroup$ – desterman Mar 28 at 11:16
  • $\begingroup$ Yes. If absorbed is light red/dark orange, the solution colour is light blue. If absorption shifts to more energetic bright-orange/yellow, the solution colour shifts to violet. $\endgroup$ – Poutnik Mar 28 at 11:18
  • $\begingroup$ Exactly. Do read the book "The Physics and Chemistry of Color - The Fifteen Causes of Color". You will benefit from the experience. $\endgroup$ – William R. Ebenezer Mar 28 at 11:19
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The supplied answers missed a very important point. Not only is there a shift in adsorption peak, the molar attenuation coefficient is much greater for the amine complex than it is for the aqua complex.

This image was taken from a paper "EXPERIMENT 7B FORMATION OF A COMPLEX ION" from Los Angeles City College at webpage:

https://www.lacitycollege.edu/Academic-Departments/Chemistry-Earth-Sciences/documents/E07B-complexion-(1).pdf

enter image description here

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  • $\begingroup$ I agreed. Sometimes, we ignore how deeply it absorbs (make big difference). Thanks for the point. $\endgroup$ – Mathew Mahindaratne Mar 28 at 18:04

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