Problem 163 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

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

Step 1: Identify the Problem

We need to determine which of the given chemical compounds does not exhibit a yellow color.

Step 2: Analyze Each Compound

Examine each given compound and its known color properties to ascertain which is not yellow. (a) \( \mathrm{Zn}_{2}[\mathrm{Fe}(\mathrm{CN})_{6}] \) - Commonly, iron cyanide complexes like this have a light blue color.(b) \( \mathrm{K}_{3}[\mathrm{Co}(\mathrm{NO}_{2})_{6}] \) - Known as a potassium cobaltinitrite, which has a yellow color.(c) \( (\mathrm{NH}_{4})_{3}[\mathrm{AS}(\mathrm{Mo}_{3}\mathrm{O}_{10})_{4}] \) - This is a yellow ammonium molybdate compound.(d) \( \mathrm{BaCrO}_{4} \) - Barium chromate is well-known for its bright yellow color.

Step 3: Conclusion

After reviewing the colors of each of the compounds, it is evident that \( \mathrm{Zn}_{2}[\mathrm{Fe}(\mathrm{CN})_{6}] \) is likely not yellow, as it is typically light blue.

Key Concepts

Color of Transition Metal Complexes

The colorful nature of transition metal complexes is a fascinating aspect of chemistry. This is due to the absorption of specific wavelengths of light, resulting in the rich colors observed. Transition metals have partially filled d-orbitals. When these metals form complexes with ligands, the d-orbital energies are split into different levels. This phenomenon is called d-d transition.

When visible light hits a transition metal complex, certain wavelengths of light are absorbed as electrons move between these split energy levels. The remaining light reflects, giving the compound its characteristic color.

• Absorption of red light results in a green color.
• Absorption of blue light results in a yellow color.
• A complex appearing blue absorbs orange light.

For example, in the exercise, the compound \(\mathrm{Zn}_{2}[\mathrm{Fe}(\mathrm{CN})_{6}]\), known for its light blue color, absorbs the orange region of the spectrum.

Coordination Chemistry

Coordination chemistry deals with the structure and behavior of complex compounds, primarily involving transition metals. These metals tend to form coordination compounds, which are composed of central metal atoms bonded to surrounding molecules or ions, known as ligands.

A typical coordination complex has a central metal cation surrounded by an array of ligands. The ligands donate electron pairs to form coordinate covalent bonds with the metal center.

• The number of coordination sites occupied by ligands is known as the coordination number.
• The geometry of the complex is influenced by the coordination number and the spatial arrangement of ligands.

In the given problem, compounds like \(\mathrm{K}_{3}[\mathrm{Co}(\mathrm{NO}_{2})_{6}]\) showcase interesting coordination chemistry where cobalt is the central metal engaging with nitrite ligands in a complex geometrical arrangement.

Ligand Field Theory

Ligand Field Theory (LFT) enhances our understanding of the electronic structure of transition metal complexes. It is an advanced reinterpretation of Coordination Theory that provides insight into why certain complexes display particular colors or magnetic properties.

According to LFT, ligands affected the energy levels of the d-orbitals in the central metal ion. The resulting split in energy levels allows us to predict the color and magnetic behavior of a complex.

• Ligands are classified according to their ability to split the d-orbital energies (known as the spectrochemical series).
• Strong field ligands, like CN⁻, cause a large splitting of the d-orbitals, which can affect color, as seen in \(\mathrm{Zn}_{2}[\mathrm{Fe}(\mathrm{CN})_{6}]\).
• Weak field ligands cause smaller energy splits, often resulting in different colors.

Understanding LFT helps predict and explain the colors observed in transition metal complexes, crucial in solving exercises involving these compounds.