Question

CN– is a strong field ligand. This is due to the fact that

• A. It is a pseudohalide
• B. It can accept electrons from metal species
• C. It forms high spin complexes with metal species
• D. It carries negative charge

Answer 1

The Correct Option is B

The question asks why CN^– is considered a strong field ligand. To answer this question, we need to understand the concept of ligands in coordination chemistry, particularly focusing on the field strength of ligands.

Coordination Chemistry Background:

In coordination chemistry, ligands are ions or molecules that bind to a central metal atom to form a coordination complex. The field strength of a ligand pertains to its ability to split the metal's d-orbitals, influencing whether a high-spin or low-spin complex is formed.

Ligands are classified as either strong field or weak field, depending on their ability to cause large or small splitting, respectively, of the metal atom's d-orbitals. This splitting is also referred to as crystal field splitting.

CN^– is classified as a strong field ligand because of its specific electronic properties.

Answer Explanation:

1. CN^– as a Strong Field Ligand: CN^– is known to be a strong field ligand due to its ability to accept electrons from the metal species. This acceptance is because CN^– can engage in π-backbonding.
2. π-Backbonding: This process occurs when a ligand like CN^– has empty π* (antibonding) orbitals that can accept electron density from filled d orbitals of the metal, effectively stabilizing the complex.
3. This electron acceptance capability results in a large crystal field splitting energy (Δ_o), categorizing CN^– as a strong field ligand, often leading to the formation of low-spin complexes.

The correct option here is: It can accept electrons from metal species, which highlights the electron accepting nature of CN^– through π-backbonding.

Other Options Analysis:

• It is a pseudohalide: While CN^– is a pseudohalide, this property is unrelated to its field strength.
• It forms high spin complexes with metal species: This is incorrect; CN^– generally forms low-spin complexes due to its strong field nature.
• It carries negative charge: The negative charge does not directly contribute to its field strength; it is about electron donation or acceptance.

Thus, the property of accepting electrons from metal species effectively explains why CN^– acts as a strong field ligand.