Question

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 1

The Correct Option is A

The question asks about the factors that contribute to the low oxidation state of metals in their complexes. The correct answer is that low oxidation states are common when ligands have good π-accepting character. Let's explore why this is the case:

1. Understanding Ligands: Ligands are ions or molecules that can donate a pair of electrons to the metal, forming a complex. The properties of ligands, such as σ-donor and π-acceptor abilities, significantly influence the oxidation state stability of the metal.
2. π-Acceptor Ligands: These ligands have empty π* orbitals that can accept electron density from the filled metal d-orbitals via back-donation. This type of interaction stabilizes lower oxidation states of the metal because:
   • π-backbonding reduces the effective electron density on the metal, making it less oxidized.
   • This stabilization occurs through synergistic bonding, where the donation of electrons from the metal to the ligand and from the ligand to the metal is balanced and enhances the stability of the complex.
3. Examples: Carbon monoxide (CO) is a classical example of a good π-acceptor ligand. Metal carbonyl complexes, such as Ni(CO)₄, exhibit metals in low oxidation states due to strong π-backbonding.
4. Ruling Out Other Options:
   • Have good σ-donor character: Pure σ-donors do not inherently favor low oxidation states as they increase electron density on the metal.
   • Have good π-donating ability: π-donors would usually increase the metal's oxidation state by adding electron density to the metal.
   • Have poor σ-donating ability: Such ligands might not effectively stabilize any particular oxidation state.
5. Conclusion: The presence of ligands with good π-accepting character enables strong π-backbonding, effectively stabilizing the metal in lower oxidation states. Hence, complexes with such ligands are more likely to feature metals in low oxidation states.