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) \(\mathrm{d}-\mathrm{d}\) transition (c) \(\mathrm{L} \rightarrow \mathrm{M}\) charge transfer transition (d) \(\sigma-\sigma^{*}\) transition

Step-by-Step Solution

Answer

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

Step 1: Understanding Charge Transfer Transitions

In a charge transfer transition, electrons are transferred between the metal and the ligand. There are two types: Metal to Ligand Charge Transfer (MLCT) and Ligand to Metal Charge Transfer (LMCT). In MLCT, the electron moves from the metal to the ligand, while in LMCT, it moves from the ligand to the metal.

Step 2: Analyze \\(\mathrm{KMnO}_{4}\\) Composition

The compound \(\mathrm{KMnO}_{4}\) consists of potassium (K), manganese (Mn), and oxygen (O). Manganese in \(\mathrm{KMnO}_{4}\) is in a high oxidation state (\(+7\)), with no d-electrons available to participate in a \(d-d\) transition. As a result, \(d-d\) transition can be ruled out.

Step 3: Determine Likely Origin of Color

Since \(d-d\) transition is not possible, examine charge transfer types. In \(\mathrm{KMnO}_{4}\), due to manganese’s high oxidation state, it is more likely for electron transfer from the oxygen ligand to manganese (LMCT) leading to absorption in the visible region and hence color formation.

Key Concepts

Charge Transfer Transition

Charge transfer transitions play a vital role in the coloration of various inorganic compounds. These transitions occur when an electron moves between a metal and a ligand. In essence, there are two main types of charge transfer transitions:

• Metal to Ligand Charge Transfer (MLCT): Here, electrons are transferred from the metal to the ligand. This transition is common in compounds where the metal is in a lower oxidation state.
• Ligand to Metal Charge Transfer (LMCT): In this transition, electrons move from the ligand to the metal, typically observed when the metal is in a high oxidation state.

These transitions are not just responsible for the color, but they also influence other properties of the compounds. Recognizing the type of charge transfer transition is crucial in predicting and explaining the behavior of a compound, such as \(\mathrm{KMnO}_{4}\). Understanding these electron movements help chemists design and manipulate compounds for desired uses, such as pigments or catalysts.

KMnO4 Color

The deep purple color of potassium permanganate \(\mathrm{KMnO}_{4}\) is intriguing and is a classic example of how ligand to metal charge transfer (LMCT) can cause vivid coloration. In \(\mathrm{KMnO}_{4}\), manganese exists in a high oxidation state of \(+7\). This elevated oxidation state means there are no d-electrons available for typical \(d-d\) transitions, which can often contribute to color in transition metals. Instead, the striking purple hue of \(\mathrm{KMnO}_{4}\) arises from LMCT, where electrons are transferred from oxygen ligands to the manganese center.
Because the manganese is in such a high oxidation state, its ability to accept electrons from the oxygen (ligand) becomes the primary mechanism for electronic transitions. This transfer happens when the compound absorbs certain wavelengths of visible light, with the remaining light being the color we perceive. Thus, in simple terms, the purple shade is the result of which wavelengths of light are absorbed and which are not, due to LMCT.

Oxidation States in Mn Compounds

Manganese is a versatile element, capable of showing a range of oxidation states, from \(+2\) to \(+7\). This property significantly influences the behavior and properties of its compounds. Here are some key points regarding oxidation states in manganese compounds:

• Low Oxidation States (e.g., \(+2\), \(+3\)): In these states, manganese can often participate in \(d-d\) transitions, contributing to the color and magnetic properties of the compounds.
• High Oxidation States (e.g., \(+6\), \(+7\)): As seen in \(\mathrm{KMnO}_{4}\), higher oxidation states typically lack d-electrons for \(d-d\) transitions. These compounds often exhibit colors resulting from ligand to metal charge transfer, as they can accept electrons from ligands.

The oxidation state imparts specific properties such as color, reactivity, and the ability to function as an oxidizing agent. In particular, higher oxidation states found in compounds like \(\mathrm{KMnO}_{4}\) are strong oxidizers, making them useful in various industrial and chemical applications. Understanding these oxidation states is essential for predicting how manganese compounds will behave in different scenarios.