Origin of Color in Transition Metal Ions & Colorless $d^0/d^{10}$$ Ions

$$\text{Colored Ions } \iff d^{1-9} \quad (\text{Presence of unpaired } d \text{-electrons enabling } d\text{--}d \text{ transitions})$$

$$\text{Colorless Ions } \iff d^0 \text{ or } d^{10} \quad (\text{No } d\text{--}d \text{ transition possible})$$

$$\begin{aligned} \text{Colorless Species: } &\quad \text{Sc}^{3+} \, (d^0), \quad \text{Ti}^{4+} \, (d^0), \quad \text{Zn}^{2+} \, (d^{10}), \quad \text{Cu}^+ \, (d^{10}) \\ \text{Characteristic Colors: } &\quad \text{Fe}^{2+} \text{ (Pale Green)}, \quad \text{Fe}^{3+} \text{ (Yellowish Brown)}, \quad \text{Cu}^{2+} \text{ (Blue)} \\ &\quad \text{Ni}^{2+} \text{ (Green)}, \quad \text{Cr}^{3+} \text{ (Violet/Green)}, \quad \text{Mn}^{2+} \text{ (Pink/Colorless)} \end{aligned}$$

Concept: When light falls on a transition metal complex, electrons are excited from lower energy $$d$$-orbitals to higher energy $$d$$-orbitals ($$d\text{--}d$$ transition). The transmitted light exhibits the complementary color of the absorbed wavelength. Ions with $$d^0$$ or $$d^{10}$$ electronic configurations cannot undergo $$d\text{--}d$$ transitions and are completely colorless.

Notation:

• $$d\text{--}d$$ transition = Excitation of an electron between split d-subshells ($$t_{2g}$$ and $$e_g$$)
• $$\text{Sc}^{3+}, \text{Ti}^{4+}, \text{Zn}^{2+}$$ = Frequently asked colorless ions