Problem 27 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following compounds is not colored yellow? (a) \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{As}\left(\mathrm{Mo}_{3} \mathrm{O}_{10}\right)_{4}\right]\) (b) \(\mathrm{BaCrO}_{4}\) (c) \(\mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (d) \(\mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right]\)

Step-by-Step Solution

Answer

\(\mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) is not yellow.

Step 1: Identify Yellow Compounds

Many chemical compounds are colored due to d-d transitions or charge transfer and can be identified by their general properties. Compounds like barium chromate (b) \( \mathrm{BaCrO}_{4} \) are known to be yellow because chromate salts are generally yellow in color.

Step 2: Compare Compounds

Now, we compare each compound given in the options:(a) \( \left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{As}\left(\mathrm{Mo}_{3}\mathrm{O}_{10}\right)_{4}\right] \) is generally yellow due to molybdenum.(b) \( \mathrm{BaCrO}_{4} \) is already identified as yellow.(c) \( \mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \) is not inherently yellow. This compound is typically a light blue or white color due to the lack of d-d transitions, as Zn and Fe are in their low oxidation states.(d) \( \mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right] \) are typically yellow as the nitrito complexes with cobalt have intense colors.

Step 3: Determine Non-Yellow Compound

From the comparison, option (c) \( \mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \) does not exhibit a yellow color like the others. Therefore, it is not colored yellow among the given options.

Key Concepts

d-d transitions

When we talk about the color of compounds, one crucial factor involved is d-d transitions. These occur in transition metal complexes when electrons in the d-orbitals absorb specific wavelengths of light. This absorption causes them to jump to a higher energy d-orbital.

The remaining light is what we perceive as color. However, not all compounds exhibit this behavior.

• Transition metals must have partially filled d-orbitals.
• Color varies with different ligands and oxidation states as they change the energy gap.

Thus, compounds like \( \mathrm{Zn}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \) are not typically yellow since zinc is not a transition metal and iron's cyanide complex has less pronounced d-d transitions.

charge transfer

Another explanation for the color in some compounds is charge transfer. This process involves the transfer of an electron from one part of the compound to another, often between the metal and a ligand.

Charge transfer can sometimes result in even more intense colors than d-d transitions as they involve more energy.

• Ligand-to-metal charge transfer (LMCT) is one common example, especially in oxidized metal-ligand systems.
• Metal-to-ligand charge transfer (MLCT) is another, occurring in reduced metal-ligand compounds.

For instance, potassium chromate \( \mathrm{BaCrO}_{4} \) owes its intense yellow color partly to charge transfer mechanisms.

molybdenum compounds

Molybdenum compounds like \( \left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{As}\left(\mathrm{Mo}_{3}\mathrm{O}_{10}\right)_{4}\right] \) are frequently yellow due to a combination of factors including their oxidation state and ligand environment.

Molybdenum often shows vibrant colors in its higher oxidation states.

• The type of oxide and its arrangement around molybdenum alters the energy gap and the observed color.
• Such properties make molybdenum compounds useful as pigments and indicators.

Hence, the appearance of yellow is a common theme in many molybdate-based chemicals.

chromate salts

Chromate salts like \( \mathrm{BaCrO}_{4} \) have a characteristic yellow color that is hard to miss. This color results largely from the chromate ion \( \text{CrO}_4^{2-} \) itself.

Often used as pigments, chromates get their color due to robust charge transfer transitions.

• Chromium in chromate is typically in a +6 oxidation state.
• The intense yellow is believed to arise from ligand-to-metal charge transfer.

Such properties are exploited in materials ranging from dyes to industrial processes requiring specific color indications.

nitrito complexes

Nitrito complexes involve nitrite ions as ligands, which can donate a lone pair to a metal center like cobalt in \( \mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)_{6}\right] \). Such complexes often exhibit vivid colors.

The formation and behavior of these complexes depend heavily on the coordination of the nitrite ion and the metal's oxidation state.

• These complexes showcase diverse stereochemistries.
• Their coordination can either be through the nitrogen or the oxygen of the nitrite ion, altering color contributions.

Moreover, the striking color variations are useful in analytical chemistry and industrial applications.