Which of the following carbonyls will have the strongest C–O bond?
- Cr(CO)6
- Fe(CO)5
- [Mn(CO)6]+
- [V(CO)6]-
Answer
(3) [Mn(CO)6]+
Solution
In metal carbonyls the metal donates electrons from a filled d orbital into the vacant antibonding π* orbital of CO (back bonding); this weakens the C–O bond.
A positive charge on the complex holds the d electrons more tightly ⇒ least back donation ⇒ C–O bond order stays highest.
A negative charge increases back donation ⇒ weakest C–O bond.
∴ C–O bond strength: [Mn(CO)6]+ > Cr(CO)6 > [V(CO)6]-; [Mn(CO)6]+ has the strongest C–O bond.
Answer: (3)