Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound is coloured due to charge transfer spectra and not due to d-d transitions? (a) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (b) \(\mathrm{KMnO}_{4}\) (c) \(\mathrm{CrO}_{3}\) (d) All of these

Step-by-Step Solution

Answer

All compounds (a), (b), and (c) are colored due to charge transfer spectra.

Step 1: Understanding Charge Transfer Spectra

Charge transfer spectra occur when an electron moves between a metal ion and a ligand, causing the compound to absorb light and appear colored. This is different from d-d transitions, where electron excitation happens within the d-orbitals of a metal ion.

Step 2: Analyzing Each Compound

We will analyze each compound to determine if their color is due to charge transfer spectra or d-d transitions. (a) **\(\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7\)**: The orange color is due to ligand-to-metal charge transfer, as there are no d-electrons in the \(\mathrm{Cr}^{6+}\) oxidation state for d-d transitions.(b) **\(\mathrm{KMnO}_4\)**: The purple color is due to charge transfer from oxygen to manganese, rather than d-d transitions, as \(\mathrm{Mn}\) is in a high oxidation state \(\mathrm{Mn}^{7+}\) with no d-electrons.(c) **\(\mathrm{CrO}_3\)**: The red color is due to charge transfer, as \(\mathrm{Cr}^{6+}\) has no d-electrons for d-d transitions to occur.

Step 3: Conclusion

Each listed compound is colored due to a charge transfer spectrum owing to the absence or lack of suitable d-electrons for d-d transitions in \(\mathrm{Cr}^{6+}\) and \(\mathrm{Mn}^{7+}\), which are present in these compounds.

Key Concepts

d-d Transitions

To understand d-d transitions, imagine a dance where electrons move between d-orbitals within the same metal ion. Transition metals, like chromium or manganese, have d-orbitals that are split into different energy levels due to the surrounding ligands. When an electron absorbs a photon, it "jumps" from a lower energy d-orbital to a higher one. This jump results in a release of energy, often perceived as color. However, not all transition metals can exhibit this dance. When an element like chromium or manganese is in a high oxidation state, such as Cr\(^{6+}\) or Mn\(^{7+}\), there may be little to no d-electrons present to engage in d-d transitions. Hence, colors in such cases arise not from this dance but from other processes, like charge transfers.

Ligand-to-Metal Charge Transfer

Ligand-to-metal charge transfer (LMCT) involves electrons transferring from a ligand to the metal center. Imagine ligands and metal ions having a give-and-take relationship, where the ligand 'gives' an electron, and the metal ion 'takes' it. This electron shift can occur, especially when the metal has a high positive charge, like Cr\(^{6+}\) or Mn\(^{7+}\), which is eager to accept electrons. When LMCT happens, the absorption of light promotes this electron movement, leading to vivid colors. For example:

• In \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), the bright orange color results from oxygen ligands transferring electrons to chromium ions.
• Similar charge transfer occurs in \(\mathrm{KMnO}_{4}\), giving it a distinct purple color, due to oxygen-to-manganese charge transfer.

These processes highlight the importance of charge transfers in determining the beautiful colors seen in many high oxidation state metal compounds.

Oxidation States in Transition Metals

Transition metals can have multiple oxidation states, which profoundly influence their chemical behavior and color. An oxidation state refers to the charge of the metal ion within a compound. The number of electrons that a metal can lose, gain, or share affects not only its reactivity but also its coloration. For instance:

• The manganese ion in \(\mathrm{KMnO}_{4}\) appears as Mn\(^{7+}\), while the chromium ion in \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) and \(\mathrm{CrO}_{3}\) occurs as Cr\(^{6+}\).
• These high oxidation states mean there are fewer or no d-electrons available for d-d transitions, shifting the role of visible light absorption to charge transfer processes.

In summary, understanding oxidation states is crucial in predicting and explaining the color of transition metal compounds. It indicates whether the color arises from typical intra-atomic d-d transitions or from alternative methods like ligand-to-metal charge transfers.