Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following complex ions, the species containing \(\mathrm{Fe}^{3+}\) in strong ligand field is (a) \(\left[\mathrm{FeF}_{6}\right]^{3-}\) (b) \(\left[\mathrm{Fe}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) (c) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) (d) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\)

Step-by-Step Solution

Answer

(c) \([\mathrm{Fe}(\mathrm{CN})_{6}]^{3-}\) has \(\mathrm{Fe}^{3+}\) in a strong ligand field (due to \(\mathrm{CN}^-\)).

Step 1: Understand the Problem

We need to identify which complex ion contains \( \mathrm{Fe}^{3+} \) in a strong ligand field. Ligands can be classified according to their field strength, and strong field ligands can induce pairing of electrons in metal ions.

Step 2: List Ligand Field Strengths

Strong field ligands are typically those that induce a large splitting of the \(d\)-orbitals in the metal ion. Common strong field ligands include cyanide \((\mathrm{CN}^-)\), carbonyl \((\mathrm{CO})\), and ammonia \((\mathrm{NH}_3)\). Weaker ligands include fluorine \((\mathrm{F}^-)\) and water \((\mathrm{H}_2\mathrm{O})\).

Step 3: Analyze Ion Choices

Assess each ion:- \(\left[\mathrm{FeF}_{6}\right]^{3-}\): \(\mathrm{F}^-\) is a weak field ligand- \(\left[\mathrm{Fe}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\): \(\mathrm{NH}_3\) is a moderate field ligand- \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\): \(\mathrm{CN}^-\) is a strong field ligand- \(\left[\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_{6}\right]^{3+}\): \(\mathrm{H}_2 \mathrm{O}\) is a weak field ligand

Step 4: Identify the Correct Species

The species \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) contains \(\mathrm{Fe}^{3+}\) in conjunction with a strong field ligand \(\mathrm{CN}^-\). This strong field causes a large splitting of \(d\)-orbitals and potential electron pairing in \(\mathrm{Fe}^{3+}\). This is the species we are looking for.

Key Concepts

Ligand Field Strength

In Crystal Field Theory, **ligand field strength** refers to the ability of a ligand to influence the splitting of the metal ion's 21d21 orbitals. When ligands approach a metal ion, they interact with its electrons, creating different energy levels for the 21d21 orbitals. The extent of this energy level difference is called d-orbital splitting.

Ligands can be categorized based on how strongly they affect the d-orbital splitting:

• Strong field ligands cause a significant splitting.
• Weak field ligands result in a smaller splitting.

The field strength of a ligand is crucial as it can affect the electronic configuration of the metal ion, potentially leading to changes in the properties and reactivity of the metal complex.

d-Orbital Splitting

The concept of **d-orbital splitting** is key to understanding how metal complexes form.

In metal ions, particularly transition metals, the 21d21 orbitals are usually degenerate, meaning they have the same energy. However, when the metal ion forms complexes with different ligands, these orbitals can split into different energy levels. This splitting is influenced primarily by:

• The type of ligand (strong or weak field ligands).
• The geometry of the metal-ligand bonding (octahedral, tetrahedral, etc.).

Energy differences in the d-orbitals can lead to variations in the magnetic and spectral properties of the compounds. Larger d-orbital splitting can cause electrons to pair up, affecting the complex's color and magnetism.

Strong Field Ligands

**Strong field ligands** play a critical role in modifying the properties of metal complexes by imposing a large energy difference between the d-orbitals.

They tend to cause a considerable splitting in the energy of the d-orbitals, leading many electrons in the metal to pair up. These ligands include cyanide (1CN12B), carbonyl (1CO1), and even ammonia (1NH1231), which, although moderate, can also act as a strong field ligand in certain situations.
Effects of strong field ligands include:

• Transition of electrons to lower energy orbitals, minimizing repulsion and stabilizing the complex.
• Often result in diamagnetic complexes (all electrons are paired).
• Can alter the electronic configuration of the metal center, potentially impacting its color and chemistry.

Understanding the nature and effect of ligands is essential for predicting the behavior of metal complexes in many chemical processes.