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 of these

Step 1: Understanding the nature of color in compounds

The color in compounds can arise due to different mechanisms, notably d-d transitions and charge transfer. In d-d transitions, color results from electronic transitions between d-orbitals. Charge transfer spectra arise when there is transfer of an electron from one atom/ion to another, often between a ligand and a central metal atom.

Step 2: Analyzing each option

We need to determine which of the given compounds exhibits color primarily from charge transfer spectra. In 1. \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), chromium exhibits a +6 oxidation state and the color is due to charge transfer. 2. \( \mathrm{KMnO}_{4}\), permanganate ion is known for its deep purple color due to charge transfer from O to Mn. 3. \( \mathrm{CrO}_{3}\) is another compound where the Cr in +6 oxidation state contributes to color through charge transfer. All these compounds exhibit charge transfer spectra dominantly.

Step 3: Conclusion

All given compounds possess color due to charge transfer spectra and not due to d-d transitions, since in each case, the metal is in a high oxidation state with no d-electrons available for traditional d-d transitions.

Key Concepts

d-d Transitions

In the world of chemistry, d-d transitions are a fundamental concept to understand when it comes to the coloration of compounds. These transitions occur within the d-orbitals of transition metal ions. Transition metals have partially filled d-orbitals, which can host electrons.
When these electrons absorb energy, they jump from a lower energy d-orbital to a higher energy d-orbital. This movement is called a d-d transition. The specific wavelengths of light absorbed during this process determine the color we observe. For instance, if a compound absorbs blue light, it will appear orange, the complementary color.

• d-d transitions are most common in metal complexes with unfilled or partially filled d-orbitals.
• The intensity of color from d-d transitions is often weaker compared to charge transfer spectra, making it sometimes difficult to observe with the naked eye.

In compounds where the metal is in a high oxidation state, like in CrO₃, KMnO₄, and K₂Cr₂O₇, no d-d transition contributes to color, as these states often lack d-electrons.

Oxidation States

Oxidation states play a crucial role in the color of a compound, especially in transition metal compounds. An oxidation state is a hypothetical charge an atom would have if all bonds were ionic.
Transition metals can have multiple oxidation states, which impact their electronic configuration significantly. These changes can alter how a compound interacts with light.

• High oxidation states often result in compounds exhibiting charge transfer spectra rather than d-d transitions.
• For instance, in compounds like \(\mathrm{CrO}_3\), \(\mathrm{Cr}\) is in a +6 oxidation state, a scenario where charge transfer is dominant.
• In \(\mathrm{KMnO}_4\), \(\mathrm{Mn}\) has an oxidation state of +7. Here, electrons are transferred from oxygen directly to manganese.

Understanding the impact of different oxidation states helps chemists predict the types of electronic transitions that will dominate and potentially influence the observed color.

Electronic Transitions

Electronic transitions form the backbone of how we understand the interaction between light and matter in chemistry. These transitions occur when an electron moves from one energy level to another, absorbing or emitting energy in the form of light.
There are several types of electronic transitions:

• d-d Transitions: These occur within d-orbitals and are common in compounds with transition metals, provided there are enough d-electrons available.
• Charge Transfer Transitions: These involve the movement of an electron from a donor atom or ligand to a metal center, typically observed in high oxidation states.

In the case of charge transfer spectra, these transitions can carry significant intensity because they involve a full transfer of electrons over a longer distance compared to d-orbital transitions.
When we say a compound such as \(\mathrm{KMnO}_4\) shows color due to charge transfer, we mean the electron jumps from an orbital on the oxygen to the metal manganese, resulting in that vibrant deep purple hue.
Understanding these transitions greatly aids in explaining why certain compounds appear in specific colors and the underlying mechanisms behind their optical properties.