Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

In the spectrochemical series, the magnitude of the crystal field splitting is maximum for which ion? (a) \(\mathrm{F}^{-}\) (b) \(\mathrm{NO}_{2}^{-}\) (c) \(\mathrm{CN}^{-}\) (d) \(\mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

(c) \( \mathrm{CN}^{-} \) causes maximum crystal field splitting.

Step 1: Understanding the Spectrochemical Series

The spectrochemical series is a list of ligands ordered by the strength of the crystal field splitting they produce when bound to a metal ion. Crystal field splitting occurs in transition metal complexes, where different ligands cause varying levels of energy difference between the d-orbitals of the central metal.

Step 2: Review the Ligands in the Options

The ligands we need to consider are (a) \(\text{F}^{-}\), (b) \(\text{NO}_{2}^{-}\), (c) \(\text{CN}^{-}\), and (d) \(\text{Cl}^{-}\). Compare these ligands to their positions in the spectrochemical series to determine which one creates the greatest crystal field splitting.

Step 3: Identify the Position of Each Ligand in the Series

In the spectrochemical series, the ligands are often ordered from weak field ligands (causing small splitting) to strong field ligands (causing large splitting). Typically, \(\text{F}^{-}\) and \(\text{Cl}^{-}\) are weak field ligands, while \(\text{NO}_{2}^{-}\) and especially \(\text{CN}^{-}\) are strong field ligands.

Step 4: Determine the Maximum Splitting Ligand

Among the given ligands, \(\text{CN}^{-}\) is recognized for being one of the strongest field ligands, leading to large crystal field splitting in compounds. \(\text{CN}^{-}\) usually appears near the very end of the spectrochemical series because of its ability to induce maximum splitting.

Key Concepts

Crystal Field Splitting

In transition metal complexes, the d-orbitals of the central metal atom or ion are split into different energy levels. This is called crystal field splitting. The degree of splitting depends on the type of ligands attached to the metal ion and their arrangement. When ligands approach a metal ion, they create an electrostatic field that affects the metal's d-orbitals. The ligands' electric field causes some orbitals to increase in energy while others decrease. This results in a splitting of what was originally a degenerate (equally energetically) set of orbitals.

The amount of splitting varies depending on the ligands' strength. Strong field ligands, like \( ext{CN}^{-}\), typically produce a larger energy difference between the split d-orbitals compared to weak field ligands like \( ext{F}^{-}\). Larger splitting results in different physical and chemical characteristics, influencing properties such as color, magnetism, and the stability of the complex.

Understanding this concept helps in explaining how various ligands affect the overall energy and behavior of transition metal complexes.

Transition Metal Complexes

Transition metal complexes consist of a central transition metal ion surrounded by molecules or ions called ligands. These ligands form coordinate bonds with the metal ion. The d-orbitals of the central metal in transition metals have unique properties due to their partially filled d-shells. This makes them ideal for forming complex structures with diverse types of ligands.

These complexes exhibit properties like variable oxidation states, colored compounds, and are often paramagnetic. This is directly related to the crystal field splitting caused by the ligands. Different ligands can alter the d-orbitals significantly, leading to complexes with widely varying properties.

• Color: The difference in energy levels created by crystal field splitting dictates the color of the complex.
• Magnetism: Depending on whether the orbitals are filled with paired or unpaired electrons, the complex can be diamagnetic or paramagnetic.
• Reactivity: The geometric arrangement and distribution of electrons influence the complex's chemical reactivity.

Transition metal complexes are crucial in many biological and industrial processes, and understanding them enhances comprehension of these broader scientific concepts.

Ligands

Ligands are ions or molecules that can donate a pair of electrons to a central metal atom or ion in a complex. They play a pivotal role in determining the structure, stability, and properties of metal complexes. Ligands can be categorized based on the number of donor atoms they possess:

• Monodentate ligands: These have one donor atom. For example, Cl\(^{-}\) and NH\(_3\).
• Bidentate ligands: These have two donor atoms, forming two bonds with the central metal, like ethylenediamine.
• Polydentate ligands: These contain multiple donor atoms, allowing for the formation of more complex and stable structures, such as EDTA.

The type and number of ligands influence the crystal field splitting significantly. While weak field ligands induce minimal splitting, strong field ligands like \( ext{CN}^{-}\) create large splitting, impacting the metal's electronic configurations and the complex's characteristics.

Understanding how different ligands interact with transition metal ions helps in predicting the resultant complex’s properties and potential applications in various fields.