JEE Main 2022 · 27 July · evening · Single correct · +4 / −1
Low oxidation state of metals in their complexes are common when ligands :
- (a) have good π-accepting character
- (b) have good σ-donor character
- (c) are having good π-donating ability
- (d) are having poor σ-donating ability
Answer and solution
Answer (a) have good π-accepting character
Solution
Chemistry ma’am · 3 steps
A low-oxidation-state metal is rich in electrons, so it needs a ligand that can take some of them back.
- The problem. A metal in a low oxidation state, such as Ni(0) in [Ni(CO)₄], holds a lot of electron density. Every ligand also pushes electrons onto it through its σ bond, so the extra charge has to go somewhere.
- Back bonding. A π-acceptor ligand like CO has empty π^* orbitals. The metal's filled d orbitals push electron density back into them, which is called back bonding, and this takes the excess charge off the metal.
Ligand character Effect on electron density at the metal Stabilises low oxidation state? Good π-acceptor Removes density by back bonding Yes Good σ-donor Adds density No Good π-donor Adds even more density No, it favours high oxidation states Poor σ-donor Adds less, but removes nothing Not by itself - So. Low oxidation states are common with ligands that have good π-accepting character, such as CO, CN⁻, NO and PR₃. The answer is (a).
The idea to keep. Electron-rich metal (low oxidation state) pairs with π-acceptor ligands like CO. Electron-poor metal (high oxidation state) pairs with π-donors like O²⁻ and F⁻.
Answer from the NTA final answer key. Solution by PrepWiser.