Question

In metal carbonyl, IR absorption can distinguish nature of CO because:

• A. absorption for CO Terminal is at high frequency than CO Bridging
• B. absorption for CO Terminal is at same frequency than CO Bridging
• C. absorption for CO Terminal is at lower frequency than CO Bridging
• D. The intensity of peak for bridging is stronger than Terminal one

Hint:

Higher bond order = Higher frequency. Terminal bonds are "triple-bond-like," so they vibrate faster!

Answer 1

The Correct Option is A

Step 1: Concept Infrared (IR) spectroscopy measures bond vibrations; the frequency depends on bond strength.

Step 2: Meaning Terminal CO bonds and Bridging CO bonds have different degrees of back-bonding from the metal.

Step 3: Analysis In bridging CO, the electron density is shared between two metals, leading to more back-donation into the $\pi^*$ orbital of CO, which weakens the C-O bond more than in terminal CO. A weaker bond vibrates at a lower frequency.

Step 4: Conclusion Thus, terminal CO (stronger bond) appears at a higher frequency (approx. 2000-2100 $cm^{-1}$) than bridging CO (approx. 1800-1900 $cm^{-1}$). Final Answer: (A)