Explain the color in coordination compounds.

(N/A) The color of coordination compounds is a major property of transition metal complexes. When white light passes through a sample,some part of it is absorbed. Therefore,the transmitted light is not white.
The color of the complex is the complementary color of the light absorbed. The complementary color is the color generated by the remaining wavelengths. For example,if green light is absorbed by the complex,it appears red. The relationship between the absorbed wavelength and the observed color is well-defined.
The color in coordination compounds can be explained in terms of Crystal Field Theory $(CFT)$.
Consider the complex $[Ti(H_2O)_6]^{3+}$,which is purple in color. This complex has a single electron $(3d^1)$ in the ground state occupying the $t_{2g}$ orbital. The next higher energy level available for the electron is the empty $e_g$ orbital.
If the complex absorbs light corresponding to the blue-green region,the electron is excited from the $t_{2g}$ level to the $e_g$ level.
$(t_{2g}^1 e_g^0 \rightarrow t_{2g}^0 e_g^1)$
As a result,the complex appears purple. This phenomenon is known as $d-d$ transition.