Question

Given below are two statements:
Statement I: The M – C \(\sigma\) bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant d-orbital of the metal
Statement II: The M – C \(\pi\) bond is formed by the donation of a pair of electrons from a filled d-orbital of metal into the vacant antibonding \(\pi\)* orbital of carbon monoxide.
In the light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are correct
• B. Both Statement I and Statement II are incorrect
• C. Statement I is correct but Statement II is incorrect
• D. Statement I is incorrect but Statement II is correct

Hint:

Visualize synergic bonding as a "give and take" relationship.

• \textbf{Ligand gives (\(\sigma\) bond):} CO donates its lone pair to the metal's empty orbital.
• \textbf{Metal gives back (\(\pi\) bond):} The metal donates electrons from its filled d-orbital to CO's empty \(\pi\)* orbital.

This dual bonding mechanism is why CO is a strong field ligand and forms very stable complexes.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question deals with the nature of bonding in metal carbonyls, characterized by a unique double-interaction known as Synergic Bonding.
Step 2: Key Formula or Approach:
Define the theoretical basis of the synergic mechanism: sigma donation from ligand to metal and pi back-donation from metal to ligand.
Step 3: Detailed Explanation:
1. Statement I: The carbon atom of the Carbon Monoxide ($CO$) ligand acts as a Lewis base. It donates its non-bonding lone pair of electrons into an empty d-orbital (or hybridized orbital) of the transition metal to form a dative $\sigma$ bond. (Statement I is True)

2. Statement II: Simultaneously, the transition metal, which now has increased electron density, acts as a Lewis base. It donates a pair of electrons from a filled, properly aligned d-orbital back into the vacant $\pi^{*}$ (antibonding) molecular orbital of the $CO$ ligand. This "back-donation" creates a $\pi$ bond. (Statement II is True)
This dual interaction creates a self-strengthening cycle (synergism) making the metal-carbon bond exceptionally strong.
Step 4: Final Answer:
Both statements accurately describe the synergic bonding mechanism.