Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The color of \( \mathrm{KMnO}_{4} \) is due to the \( \mathrm{L} \rightarrow \mathrm{M} \) charge transfer transition.

Step 1: Understanding the problem

We need to determine the reason for the color in the compound \( \mathrm{KMnO}_{4} \). It has four possible reasons given: \( \mathrm{M} \rightarrow \mathrm{L} \) charge transfer transition, \( d-d \) transition, \( \mathrm{L} \rightarrow \mathrm{M} \) charge transfer transition, and \( \sigma - \sigma^{*} \) transition.

Step 2: Examining the potential transitions

The compound \( \mathrm{KMnO}_{4} \) is known as potassium permanganate, where manganese is in a high oxidation state (+7). In this state, \( d-d \) transitions and \( \sigma - \sigma^{*} \) transitions are unlikely because \( \mathrm{KMnO}_{4} \) has no unpaired electrons for a \( d-d \) transition to occur and \( \sigma - \sigma^{*} \) transitions are not typical for this type of compound.

Step 3: Determining the correct type of charge transfer

Between \( \mathrm{M} \rightarrow \mathrm{L} \) (metal to ligand) and \( \mathrm{L} \rightarrow \mathrm{M} \) (ligand to metal), given the high oxidation state, \( \mathrm{L} \rightarrow \mathrm{M} \) charge transfer is more likely. This involves electrons moving from the ligand oxygen atoms to the metal manganese, which is responsible for the color in \( \mathrm{KMnO}_{4} \).

Key Concepts

Charge Transfer Transition

Charge transfer transitions are fascinating phenomena in inorganic chemistry that contribute significantly to the color of many compounds. They occur when electrons are transferred between a metal atom and the ligands bound to it. In potassium permanganate (\(\mathrm{KMnO}_{4}\)), the color results from a particular type of charge transfer known as \(\mathrm{L} \rightarrow \mathrm{M}\) transition.

In this process, electrons move from the oxygen atoms in the permanganate ion (ligands) to the manganese atom.

• Metal to Ligand (\(\mathrm{M} \rightarrow \mathrm{L}\)) transfer: Electrons are transferred from the metal to the ligands.
• Ligand to Metal (\(\mathrm{L} \rightarrow \mathrm{M}\)) transfer: Electrons are transferred from the ligands to the metal, as seen in \(\mathrm{KMnO}_{4}\).

This transfer is energetically favorable, especially in compounds where the metal is in a high oxidation state, like manganese in potassium permanganate. As the electron transfer happens, it absorbs specific wavelengths of light, which results in the vibrant color characteristic of these compounds. Thus, the beautiful purple hue of potassium permanganate is primarily due to these \(\mathrm{L} \rightarrow \mathrm{M}\) charge transfer transitions.

Manganese Oxidation States

Manganese is an element that exhibits a variety of oxidation states, ranging from -3 to +7. These different states significantly alter the chemical and physical properties of manganese-containing compounds. In potassium permanganate (\(\mathrm{KMnO}_{4}\)), manganese is in its highest oxidation state of +7, which directly influences the color of the compound.

This high oxidation state means that manganese has lost seven electrons, making it a powerful oxidizing agent. Here’s a brief look at the significance of manganese oxidation states:

• Low oxidation states (0 to +2): Typically give rise to compounds that are less energetically inclined to participate in electron transfer.
• High oxidation states (+6 and +7): These states are more likely to engage in significant electron-transfer processes like ligand to metal charge transfers that are responsible for the color of \(\mathrm{KMnO}_{4}\).

The +7 oxidation state of manganese in potassium permanganate not only contributes to its chemical reactivity but also plays a crucial role in its intense coloration. This makes understanding oxidation states important, not just for chemistry, but also to appreciate the diverse applications of manganese-based compounds.

Color in Inorganic Compounds

Inorganic compounds display a wide array of colors due to their electronic structures and the transitions between different energy levels. These colors arise mainly from charge transfer transitions and \(d-d\) transitions. However, in some cases, simple electron pairing or arrangement can affect the compound's overall hue.

Potassium permanganate (\(\mathrm{KMnO}_4\)) is an excellent example of how electronic interactions produce vivid colors. The intense purple color is due to the \(\mathrm{L} \rightarrow \mathrm{M}\) charge transfer transition:

• Other compounds, particularly those with metal centers and coordinated ligands, can also exhibit colors due to similar charge transfers or \(d-d\) transitions.
• The specific colors observed are due to the wavelengths of light absorbed during these electronic transitions.

As electrons jump to higher energy levels, they absorb particular wavelengths, and the complementary colors are what we observe. For students, understanding these processes can illuminate the underlying principles behind the colors of various inorganic compounds. From art to chemistry, these transitions bridge the gap between scientific understanding and aesthetic appreciation.