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Color of Transition Metal Complexes
The variety of color among transition metal complexes has long fascinated the chemists. For example, aqueous solutions of $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ are red, $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ are pink, $\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ are green, $\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ are blue and $\left[\mathrm{Zn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ are colorless. Although the octahedral $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ are pink, those of tetrahedral $\left[\mathrm{CoCl}_{4}\right]^{2-}$ are blue. The green color of $\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ turns blue when ammonia is added to give $\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}$. Many of these facts can be rationalized from CFT.
Vanadium metal (center) and in solution as $\mathrm{V}^{2+}(a q), \mathrm{V}^{3+}(a q), \mathrm{VO}^{2+}(a q)$, and $\mathrm{VO}_{2}{ }^{+}(a q)$, (left to right).
Why we see Color?
When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes. If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample, and then the sample appears black. If the sample absorbs no visible light, it is white or colorless. When the sample absorbs a photon of visible light, it is its complementary color we actually see. For example, if the sample absorbed orange color, it would appear blue; blue and orange are said to be complementary colors.
The Color Wheel
The visible part of the electromagnetic spectrum contains light of wavelength 380-750 nm . The color wheel below gives information on the wavelength of different color and also the complementary color. For example: If red
light is absorbed, the complex appears green; If
Colour of light absorbed | Approx. $\lambda$ ranges / nm | Colour of light transmitted
Red | 700-620 | Green
Orange | 620-580 | Blue
Yellow | 580-560 | Violet
Green | 560-490 | Red
Blue | 490-430 | Orange
Violet | 430-380 | Yellow