Question

In the structure of the dichromate ion, there is a :

• A. linear symmetrical \( \text{Cr}-\text{O}-\text{Cr} \) bond.
• B. linear unsymmetrical \( \text{Cr}-\text{O}-\text{Cr} \) bond.
• C. non-linear symmetrical \( \text{Cr}-\text{O}-\text{Cr} \) bond.
• D. non-linear unsymmetrical \( \text{Cr}-\text{O}-\text{Cr} \) bond.

Hint:

Dichromate ions are stable in acidic medium, whereas chromate ions (\( \text{CrO}_4^{2-} \)) are stable in basic medium. The transition occurs via the \( \text{Cr}-\text{O}-\text{Cr} \) bridge formation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The dichromate ion \( (\text{Cr}_2\text{O}_7^{2-}) \) is formed by the sharing of a corner oxygen atom between two tetrahedral \( \text{CrO}_4 \) units.
Step 2: Detailed Explanation:
1. Bond Angle: In the \( \text{Cr}_2\text{O}_7^{2-} \) ion, the bridging \( \text{Cr}-\text{O}-\text{Cr} \) bond angle is approximately \( 126^\circ \). Because this angle is not \( 180^\circ \), the bond is non-linear (bent).
2. Symmetry: The two chromium atoms are in the same environment, and the bridging \( \text{Cr}-\text{O} \) bond lengths are equal (symmetrical).
3. Structure Details: The bridging \( \text{Cr}-\text{O} \) bond length is \( 179 \, \text{pm} \), which is longer than the terminal \( \text{Cr}-\text{O} \) bond lengths (\( 163 \, \text{pm} \)) due to the absence of resonance in the bridge.
Step 3: Final Answer:
The structure contains a non-linear symmetrical \( \text{Cr}-\text{O}-\text{Cr} \) linkage.