Question

The light pink colour of \([Co(H_2O)_6]^{2+}\) and the deep blue colour of \([CoCl_4]^{2-}\) are due to:

• A. MLCT transition in the first and d-d transition in the second
• B. LMCT transition in both
• C. d-d transitions in both
• D. d-d transition in the first and MLCT transition in the second

Hint:

Centrosymmetric complexes (like octahedral) have pale colors due to Laporte forbidden transitions. Non-centrosymmetric complexes (like tetrahedral) have intense colors.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Colors in transition metal complexes predominantly arise from d-d transitions or Charge Transfer transitions.
Step 2: Detailed Explanation:
1. Hexaaquacobalt(II): An octahedral complex where d-d transitions are Laporte-forbidden. This results in weak absorption and a pale pink color.
2. Tetrachlorocobaltate(II): A tetrahedral complex where the absence of a center of inversion relaxes the Laporte rule, making the d-d transitions much more intense and leading to a deep blue color.
Both colors are caused by the excitation of an electron between split d-orbitals.
Step 3: Final Answer:
The transitions are d-d in both complexes.