Question

Complex \([\text{Co}(\text{H}_{2}\text{O})_{6}]^{2+}\) when dissolved in water forms a pink solution. But when this solution is treated with concentrated hydrochloric acid, it turns deep blue to form another complex \([\text{CoCl}_{4}]^{2-}\). Identify the main reason for this colour change.

• A. presence of unpaired electrons
• B. charge transfer
• C. nature of ligands attached to the metal ion
• D. change in geometry of new complex formed

Hint:

Compare the shape of the aqua complex with that of the chloro complex formed in conc. HCl.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
The colour of a complex depends on the crystal field splitting energy, which depends on geometry and the ligands.

Step 2: Compare the two complexes
\([\text{Co}(\text{H}_{2}\text{O})_{6}]^{2+}\) is octahedral and pink.
\([\text{CoCl}_{4}]^{2-}\) is tetrahedral and deep blue.

Step 3: Splitting energy
Tetrahedral splitting is much smaller than octahedral, \(\Delta_{t}=\dfrac{4}{9}\Delta_{o}\), so the absorbed wavelength changes.

Step 4: Conclusion
The main reason given among the options is the change in geometry of the new complex.

Final Answer:
The colour changes mainly because the geometry changes from octahedral to tetrahedral. \[ \boxed{\text{(D) }\text{change in geometry of new complex formed}} \]