How to predict the reason for color in a transition metal complex?
How to predict the reason for color in a transition metal complex?

Nobody can do it just by &quot;looking&quot; at the color of any compound. Spectroscopists have to do calculations to figure that out. There are at least 15 reasons for the &quot;color&quot; of chemical compounds.
Now qualitatively one can &quot;predict&quot; whether it is a d-d vs. it is charge transfer transition for transition metal compounds.
Keep in mind that d-d transitions are &quot;forbidden&quot; (Laporte selection rule). Therefore, majority of the d-d transitions correspond to low molar absorptivity. For example, Cu(II) solution is light blue, manganese (II) is almost invisible pink, chromium (III) is light green.
Charge transfer transitions are very intense! Add two three crystals of potassium permanganate to water, and it is intensely purple.