Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The color of \(\mathrm{KMnO}_{4}\) is due to : (a) \(\mathrm{L} \rightarrow \mathrm{M}\) charge transfer transition (b) \(\sigma-\sigma^{*}\) transition (c) \(\mathrm{M} \rightarrow \mathrm{L}\) charge transfer transition (d) \(\mathrm{d}-\mathrm{d}\) transition

Step-by-Step Solution

Answer

The color of \( \mathrm{KMnO}_4 \) is due to \(\mathrm{L} \rightarrow \mathrm{M}\) charge transfer transition.

Step 1: Identify the Compound

The compound given in the problem is potassium permanganate, with the chemical formula \( \mathrm{KMnO}_4 \). It is known for its deep purple color.

Step 2: Understand Charge Transfer Transition

In transition metal compounds, colors are often due to electronic transitions between different energy levels. Among these types of transitions, charge transfer transitions involve the movement of an electron between the metal and the ligand.

Step 3: Determine the Type of Charge Transfer

Consider the components of \( \mathrm{KMnO}_4 \). The manganese ion in this compound exists in a high oxidation state \((+7)\), which is quite effective in charge transfer processes involving electrons from the oxygen ligands. This gives rise to intense color due to a ligand to metal charge transfer (\( \mathrm{L} \rightarrow \mathrm{M} \) transition).

Step 4: Analyze Other Transition Possibilities

Other transition options include \( \sigma-\sigma^{*} \) transitions, \( \mathrm{M} \rightarrow \mathrm{L} \) charge transfer transitions, and \( \mathrm{d}-\mathrm{d} \) transitions. \( \mathrm{d}-\mathrm{d} \) transitions typically occur in lower oxidation states and are weak compared to charge transfer transitions. Since \( \mathrm{KMnO}_4 \) doesn't have partially filled \( d \)-orbitals, a \( \mathrm{d}-\mathrm{d} \) transition isn't feasible. Similarly, \( \mathrm{M} \rightarrow \mathrm{L} \) charge transfer doesn't suit the highly positively charged \( \mathrm{Mn}^{7+} \). Thus, \( \mathrm{L} \rightarrow \mathrm{M} \) is the most appropriate option.

Key Concepts

Charge Transfer Transition

Charge transfer transitions are intriguing phenomena where an electron moves from one part of a molecule to another, greatly affecting its properties, such as color.
In the case of potassium permanganate (\(\text{KMnO}_4\)), the deep purple color arises mainly from such a transition.
This occurs when electrons are transferred between the oxygen ligands and the manganese metal center.The beauty of a charge transfer transition is its ability to create vibrant colors, as it involves significant energy changes:

• Electrons move between a ligand and a metal (or vice versa), inducing shifts in energy levels.
• This shift corresponds to wavelengths in the visible spectrum, which, in turn, produce color.

The impressive hue of \(\text{KMnO}_4\) is primarily a result of the ligand to metal charge transfer transition, where electrons from the oxygen ligands transfer to the manganese ion.

Electronic Transitions

Electronic transitions are key to understanding the coloration of compounds.
These transitions occur when electrons absorb energy and jump to a higher energy level.
In transition metal complexes, electrons can move between different orbitals, each promoting a different type of transition.There are several kinds of electronic transitions you might encounter:

• Charge Transfer Transitions: Involve an electron moving from a ligand orbital to a metal orbital or vice versa.
• d-d Transitions: Typically occur in compounds with partially filled d-orbitals, though absent in \(\text{KMnO}_4\) due to the high oxidation state of manganese (\(\text{Mn}^{7+}\)).
• σ-σ* and π-π* Transitions: Occur in general molecules but are not predominant in transition metal complexes.

In the context of \(\text{KMnO}_4\), it's the charge transfer transitions that primarily account for its characteristic color, not the other types.

Ligand to Metal Charge Transfer

Ligand to metal charge transfer (LMCT) is a specific type of electronic transition.
It plays a significant role in the colorful display of complexes such as \(\text{KMnO}_4\).
This process involves electron donation from the ligand's electrons to unfilled metal orbitals.Why does LMCT occur in \(\text{KMnO}_4\)?

• In \(\text{KMnO}_4\), oxygen ligands serve as electron donors, while the \(\text{Mn}^{7+}\) ion acts as an electron acceptor.
• The manganese ion is in a high oxidation state, making it very electron deficient and an ideal recipient for electrons from the ligands.

When electrons move from an oxygen ligand to the manganese metal center, a LMCT transition happens, resulting in the striking purple color of \(\text{KMnO}_4\). This transition involves a highly energetic process, releasing energy in the form of visible light.