Understanding Colour in Transition Metal Ions

The colour of transition metal ions is usually due to electronic transitions within the d orbitals, specifically known as d-d transitions. For these transitions to occur, the d orbitals must be partially filled. This means the transition metal ion should have between one (d¹) and nine (d⁹) electrons in its d subshell. If the d subshell is completely empty (d⁰) or completely filled (d¹⁰), d-d transitions are not possible, and the ions are typically colourless.

Analysing the Given Transition Metal Ions for Colour

Let's examine the electronic configuration of each given transition metal ion to determine the number of d electrons and predict whether it will be coloured or colourless.

Explanation of Electronic Configurations and Colour

• Scandium ion \( \text{Sc}^{3+} \): Neutral Scandium (Sc) has an electronic configuration of \( \left[ \text{Ar} \right] 3\text{d}^1 4\text{s}^2 \). When it loses three electrons to form \( \text{Sc}^{3+} \), it loses the two 4s electrons and the one 3d electron. The resulting configuration is \( \left[ \text{Ar} \right] 3\text{d}^0 \). With zero electrons in the d orbital (d⁰), there are no electrons available for d-d transitions. Therefore, \( \text{Sc}^{3+} \) is colourless.
• Vanadium ion \( \text{V}^{2+} \): Neutral Vanadium (V) has \( \left[ \text{Ar} \right] 3\text{d}^3 4\text{s}^2 \). \( \text{V}^{2+} \) loses two 4s electrons, resulting in \( \left[ \text{Ar} \right] 3\text{d}^3 \). Having three electrons in the d orbital (d³), it can undergo d-d transitions and is coloured (typically violet).
• Manganese ion \( \text{Mn}^{2+} \): Neutral Manganese (Mn) has \( \left[ \text{Ar} \right] 3\text{d}^5 4\text{s}^2 \). \( \text{Mn}^{2+} \) loses two 4s electrons, resulting in \( \left[ \text{Ar} \right] 3\text{d}^5 \). With five electrons in the d orbital (d⁵), it can undergo d-d transitions and is coloured (typically pale pink).
• Cobalt ion \( \text{Co}^{3+} \): Neutral Cobalt (Co) has \( \left[ \text{Ar} \right] 3\text{d}^7 4\text{s}^2 \). \( \text{Co}^{3+} \) loses two 4s electrons and one 3d electron, resulting in \( \left[ \text{Ar} \right] 3\text{d}^6 \). With six electrons in the d orbital (d⁶), it can undergo d-d transitions and is coloured (typically blue in simple aqueous solution, but complexes are common and vary in colour).

Based on the electronic configurations, only \( \text{Sc}^{3+} \) has a d⁰ configuration, making it unable to undergo d-d transitions and thus colourless. The other ions have partially filled d orbitals (d³, d⁵, d⁶) and are therefore coloured.

Conclusion: Colourless Transition Metal Ion

Among the given options, the transition metal ion that is colourless is \( \text{Sc}^{3+} \).

Revision Table: Transition Metal Ion Colour

Additional Information: Factors Affecting Transition Metal Colour

While d-d transitions are the primary reason for the colour of transition metal ions, other factors can also influence or cause colour:

• Ligands: The nature of ligands surrounding the metal ion in coordination complexes significantly affects the splitting of d orbitals (crystal field splitting), which in turn affects the energy difference for d-d transitions and thus the colour absorbed/transmitted.
• Charge Transfer Transitions: In some cases, colour arises from the transfer of an electron from the ligand to the metal ion (Ligand to Metal Charge Transfer, LMCT) or from the metal ion to the ligand (Metal to Ligand Charge Transfer, MLCT). These transitions are often more intense than d-d transitions and can occur even in d⁰ or d¹⁰ complexes if the energy difference is suitable. However, for simple hydrated ions of the first transition series, d-d transitions are the main cause of colour.