Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds are coloured due to charge transfer spectra? (a) \(\mathrm{AgNO}_{3}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{KMnO}_{4}\)

Step-by-Step Solution

Answer

K_2Cr_2O_7
and
KMnO_4
are colored due to charge transfer spectra.

Step 1: Understanding Charge Transfer Spectra

Charge transfer spectra occur when there is an electronic transition between a metal ion and a ligand within a coordination compound. This typically results in the compound exhibiting color. For a compound to display charge transfer spectra, it must have an appropriate electronic configuration to facilitate these transitions.

Step 2: Analyzing Compound (a): AgNO3

Silver nitrate (
AgNO_3
) is an ionic compound and not a coordination compound. It does not have the mixed-valency or ligand-to-metal charge transfer necessary to cause color due to charge transfer spectra. Therefore,
AgNO_3
is colorless.

Step 3: Analyzing Compound (b): CuSO4

Copper(II) sulfate (
CuSO_4
) exhibits a blue color, primarily due to d-d transitions rather than charge transfer. The presence of water molecules coordinated to the copper ion causes these transitions. Thus,
CuSO_4
is not colored due to charge transfer spectra.

Step 4: Analyzing Compound (c): K2Cr2O7

Potassium dichromate (
K_2Cr_2O_7
) exhibits an intense orange color that is due to ligand-to-metal charge transfer. This involves electrons transferring from oxygen atoms of the chromate ion to the chromium ions, which makes it colored due to charge transfer spectra.

Step 5: Analyzing Compound (d): KMnO4

Potassium permanganate (
KMnO_4
) exhibits a deep purple color due to charge transfer spectra. This involves electrons moving from the oxygen atoms in the permanganate ion to the manganese ion, leading to its characteristic color.

Key Concepts

Coordination Compounds

Coordination compounds are a fascinating area of chemistry that revolve around complex structures formed between metal ions and ligands. A metal ion, often a transition metal, acts as the central atom. Ligands, which can be molecules or ions, attach to this central ion, creating a coordination complex.
A key characteristic of these compounds is their unique ability to display vivid colors, thanks to their electronic arrangements. These colors arise when light interacts with the electrons in the metal-ligand bonds, causing electronic transitions. Coordination compounds are prevalent in many applications, from pigmentation in dyes to catalysis in industrial processes. Understanding how these complexes work opens the door to appreciating their role in chemistry and beyond.

Electronic Transitions

The heart of color in coordination compounds lies in electronic transitions. These refer to the movement of electrons between different energy levels or orbitals. When light strikes a coordination compound, it can excite electrons, promoting them to higher energy states.
There are different types of electronic transitions:

• d-d Transitions: Involves movement of electrons within the d orbitals of a transition metal. This is common in metal ions in a complex.


• Ligand-to-Metal Charge Transfer (LMCT): Here, electrons transfer from the ligand to the metal center.


• Metal-to-Ligand Charge Transfer (MLCT): This occurs when electrons move from the metal to the ligand.

These transitions are central to understanding why coordination complexes have the colors that they do. They can result in the absorption of specific wavelengths of light, which directly affect the color observed.

Ligand-to-Metal Charge Transfer

The Ligand-to-Metal Charge Transfer (LMCT) is an intriguing process in coordination chemistry. In LMCT, electrons move from a ligand to the metal ion in the complex. This transfer significantly contributes to the color displayed by some compounds.
For example, in potassium dichromate (\(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\)), electrons shift from the oxygen atoms in the chromate ion to the chromium ions. Such movements allow for intense color experiences, typically in the visible spectrum.

• LMCT is useful in compounds where the ligands possess higher energy electrons compared to the metal ions.


• This transition broadens the scope for designing materials with specific optical properties, making it invaluable in fields like photonics and sensor technology.

By comprehending LMCT, chemists can manipulate and predict the property changes in various compounds.

Color in Chemistry

The vibrant colors in chemistry aren't just for show; they represent the fascinating microscopic world of electrons and energy. The reason we see colors lies in the interaction between a compound's electrons and incoming light.
Specifically for coordination compounds, certain wavelengths of light are absorbed due to electronic transitions, allowing only particular colors to reflect back to our eyes.

• Metal ions with partially filled d-orbitals often display rich colors due to transitions within these orbitals or between orbitals of metal and ligand (charge transfer).


• Different metals and ligands pairings lead to diverse colors based on the energies involved in these electron transitions.
• The vibrant purple of potassium permanganate (\(\mathrm{KMnO}_{4}\)) is due to oxygen to manganese charge transfer, a notable example of charge transfer spectra efficiently creating color.

The study of how and why these colors manifest leads to understanding their utility, ranging from aesthetic applications like dyes and paints to functional uses in scientific detection methods and analyses.