Q.[Ti(H₂O)₆]³⁺ is coloured, while [Sc(H₂O)₆]³⁺ is colourless- explain.

Step 1. Colour in a transition-metal complex arises from a d-d transition: absorption of visible light promotes an electron from the lower t2g level to the higher eg level (Section 5.6.2.4).

Step 2. Ti³⁺ has configuration 3d¹ -- its single electron sits in a t2g orbital and CAN be promoted to an eg orbital by absorbing light of energy Δ₀; for [Ti(H₂O)₆]³⁺ this absorption occurs at 20000 cm⁻¹ (Δ₀ = 239.7 kJ/mol), and the transmitted light is purple, so the complex is coloured (purple).

Step 3. Sc³⁺ has configuration 3d⁰ -- there is NO d electron present at all to be promoted from t2g to eg, so a d-d transition is structurally impossible. …

Compare the d-electron count of Ti³⁺ (d¹) and Sc³⁺ (d⁰) and check whether a d-d transition (t2g to eg prom …

• Attributing the colour difference to the ligand (both are H₂O here) rather than to the metal's own d-electron count -- the ligand is identical in both complexes, so the difference must come from the metal. …