Question

Among the following metal carbonyl species, the one with the highest metal-carbon back bonding is

• A. \([ \text{Ti(CO)}_6 ]^{2-} \)
• B. \([ \text{V(CO)}_6 ]^{-} \)
• C. \([ \text{Cr(CO)}_6 ] \)
• D. \([ \text{Mn(CO)}_6 ]^{+} \)

Hint:

Back bonding is most effective when the metal has a low oxidation state and the ligand (CO) can accept electron density into its \( \pi^* \) orbitals.

Answer 1

The Correct Option is A

Understanding metal-carbon back bonding in metal carbonyls:

Back bonding occurs when:

1. Metal donates electron density from filled d-orbitals
2. Into empty π* antibonding orbitals of CO
3. This strengthens M-C bond and weakens C-O bond

Factors favoring strong back bonding:

• High electron density on metal (negative charge, low oxidation state)
• Good orbital overlap between metal d-orbitals and CO π* orbitals

Analyzing each complex:

(A) [Ti(CO)₆]²⁻

• Ti in oxidation state: -2 (very electron-rich)
• Configuration: Ti²⁻ has d⁶ configuration
• Very high electron density on metal
• Strong back donation expected
• Highest back bonding 

(B) [V(CO)₆]⁻

• V in oxidation state: -1
• Configuration: V⁻ has d⁶ configuration
• High electron density, but less than Ti²⁻
• Strong back bonding, but less than (A)

(C) Cr(CO)₆

• Cr in oxidation state: 0
• Configuration: Cr⁰ has d⁶ configuration
• Neutral metal, moderate electron density
• Good back bonding, but less than charged species

(D) [Mn(CO)₆]⁺

• Mn in oxidation state: +1
• Configuration: Mn⁺ has d⁶ configuration
• Positive charge reduces electron density
• Weakest back bonding among these

Trend: As metal becomes more electron-rich (more negative charge), back bonding increases.

Order: [Ti(CO)₆]²⁻ > [V(CO)₆]⁻ > Cr(CO)₆ > [Mn(CO)₆]⁺

Answer: (A) [Ti(CO)₆]²⁻