Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

When degenerate d-orbitals of an isolated atom/ion are brought under the impact of magnetic field of ligands, the degeneracy is lost. The two newly formed sets of d-orbitals, depending upon nature and magnetic field of ligands are either stabilized or destabilized. The energy difference between the two sets whenever lies in the visible region of the electromagnetic spectrum, then the electronic transition \(\mathrm{t}_{2 \mathrm{~g}} \rightleftharpoons \mathrm{e}_{8}\) are responsible for colours of the co- ordination compounds Which of the following colour is not due to d-d transition of (a) Yellow colour of CdS. (b) Red colour of blood (c) Orange colour of \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) in acidic medium. (d) Both (a) and (c)

Step-by-Step Solution

Answer

Both (a) and (c) are not due to d-d transition.

Step 1: Understanding d-d Transition

The term 'd-d transition' refers to the electronic transitions that occur between d-orbitals within transition metal complexes. In these cases, ligands create a crystal field that splits the degenerate d-orbitals into two groups with different energy levels. These transitions can absorb specific frequencies of light, leading to the colors observed in coordination compounds.

Step 2: Analyzing Given Compounds

We need to evaluate each compound to determine whether its color is a result of d-d transitions. Compounds that contain transition metals with partially filled d-orbitals and show evidence of d-d transitions typically exhibit this property.

Step 3: Examining CdS

CdS (Cadmium Sulfide) is not a coordination compound and does not contain transition metals with partially filled d-orbitals. The yellow color of CdS comes from its band gap, not from d-d transition.

Step 4: Examining Blood

Blood contains hemoglobin, which has iron as a central metal in a porphyrin ring. These contain iron in a d-electron configuration allowing d-d transitions involving iron, contributing to its color.

Step 5: Examining Cr₂O₇²⁻

The orange color of Cr₂O₇²⁻ in an acidic solution is due to charge transfer transitions, not traditional d-d transitions. Chromates have the Cr(VI) oxidation state which has an empty d-orbital, precluding d-d transitions.

Step 6: Conclusion from Analysis

From our analysis, options (a) Yellow colour of CdS and (c) Orange colour of Cr₂O₇²⁻ involve transitions other than d-d transitions.

Key Concepts

Degenerate d-orbitals

In transition metal ions, the d-orbitals are initially degenerate, meaning they all have the same energy level when in isolation. However, this situation changes in the presence of a magnetic field created by ligands in a coordination compound. The ligands influence these orbitals due to their electric fields.

Once these orbitals are in a complex, they typically split into groups with different energies due to differences in how they interact with the ligands. This splitting breaks the degeneracy, creating energy differences between the sets of d-orbitals. Understanding this concept is crucial for explaining many properties of transition metals, including color and magnetic behaviors.

Crystal field splitting

Crystal field splitting is the process where degenerate d-orbitals in a transition metal ion split into different energy levels. This occurs when the ion is part of a coordination compound surrounded by ligands. The ligands' electric field interacts differently with each d-orbital, based on their geometric arrangement around the metal center.

The splitting produces two groups: typically, a lower-energy set and a higher-energy set. The difference in energy between these groups is called the crystal field splitting energy, denoted as \(_0_0\Delta\). When these energy differences fall within the visible spectrum, they lead to the fascinating colors observed in many transition metal complexes.

This process forms the basis of understanding color changes and electronic behaviors in coordination compounds. The ligands' nature, the metal's oxidation state, and geometric structure heavily influence the extent of crystal field splitting.

Coordination compounds

Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ions, known as ligands. These ligands can be neutral or charged entities that donate lone pairs of electrons to the metal, forming coordinate covalent bonds.

• The central metal is usually a transition metal.
• Ligands determine the compound's geometry, stability, and reactivity.
• The number of ligands varies, often reflecting the metal's coordination number.

The nature of the ligands and the metal center greatly affects the properties of the compound, influencing characteristics like color, solubility, and magnetic behavior. Coordination chemistry plays a crucial role in various fields such as catalysis, material science, and biology, offering insight into complex substances like hemoglobin.

Transition metal complexes

Transition metal complexes are formed when transition metals bind with ligands. These complexes are notable for their diverse structures and fascinating properties.

One of the signature features of transition metals is their partially filled d-orbitals, which allow for a variety of possible oxidation states and complex formations. This attribute contributes to the wide array of colors displayed by these complexes, often due to d-d transitions.

• These complexes are highly important in industrial and chemical processes.
• They often act as catalysts or serve in electron transfer processes.
• Transition metal complexes can even exhibit unique magnetic and electronic properties.

Their importance extends beyond chemistry into biological systems, where they are essential in processes like oxygen transport and enzymatic reactions. Understanding the nature of these complexes helps you grasp the underlying principles of their pervasive roles in science and technology.