Question

Why is \([Ni(H_2O)_6]^{2+}\) coloured? (Atomic number of Ni = 28)

Hint:

Colour in transition complexes → d–d transitions in partially filled d-orbitals.

Answer 1

Concept: Transition metal complexes are coloured due to
d–d transitions. These occur when electrons absorb visible light and move between split d-orbitals.
Step 1: Electronic configuration of Ni. Ni (Z = 28): \[ [Ar] \, 3d^8 4s^2 \] Ni\(^{2+}\): \[ 3d^8 \]
Step 2: Effect of ligand field. In octahedral complex: \[ d \text{-orbitals split into } t_{2g} \text{ and } e_g \]
Step 3: Reason for colour.

• Partially filled d-orbitals present
• d–d electronic transitions occur
• Absorption of visible light → complementary colour observed

Conclusion: \[ \therefore [Ni(H_2O)_6]^{2+} \text{ is coloured due to d–d transitions. \]