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

Question

Which of the following compounds is not colored yellow? \([2015]\) (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) \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{AS}\left(\mathrm{Mo}_{3} \mathrm{O}_{\mathrm{H} 0}\right)_{4}\right]\) (d) \(\mathrm{BaCrO}_{4}\)

Step-by-Step Solution

Answer

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

Step 1: Understand the Color Indication

Compounds with specific transition metals can show colors due to electronic transitions. Not all compounds of transition metals are colored, and not all colored compounds are yellow. The task is to identify which of these compounds is not yellow.

Step 2: Analyze Each Compound

(a) \(\text{Zn}_2[\text{Fe(CN)}_6] \) - This is a complex which usually shows colors due to \(\text{Fe}^{3+} \), but it is generally whitish-gray in appearance, indicating it is not yellow.(b) \(\text{K}_3[\text{Co(NO}_2)_6] \) - This cobalt complex is typically yellow due to the characteristic coloration of cobalt nitrite complexes.(c) \((\text{NH}_4)_3[\text{As(Mo}_3\text{O}_9)_4] \) - Also known as ammonium phosphomolybdate, is yellow because it contains molybdenum oxides.(d) \(\text{BaCrO}_4 \) - This barium chromate compound is yellow because it is a chromate, which are often yellow or orange due to the chromate ion.

Step 3: Determine the Odd Compound

Having analyzed each compound, \(\text{Zn}_2[\text{Fe(CN)}_6]\) is not yellow, while the rest are known for their yellow coloration. This is due to the colorless nature of zinc complexes and the specific metal-to-ligand charge transfer properties in \(\text{Fe}\) complexes, leading to a grayish or colorless compound, rather than yellow.

Key Concepts

Transition Metals

Transition metals are elements found in the d-block of the periodic table, often recognized for their ability to form colorful compounds and their unique electronic configurations. One of the key characteristics of transition metals is their partially filled d orbitals, which allow them to form various oxidation states and complex geometries.
These metals are capable of forming coordination complexes by bonding with ligands—molecules or ions that donate electron pairs to the metal center. This ability to form multiple stable complexes is a result of the flexible nature of their d orbitals. Understanding transition metals is crucial when exploring the properties and appearances of coordination complexes, as seen in the compounds discussed in the exercise. For instance:

• Zinc, traditionally not considered a transition metal, cannot undergo d-d transitions due to its full d orbital (d¹⁰ configuration).
• Iron and cobalt, by contrast, have partially filled d orbitals, enabling them to participate in electronic transitions responsible for the observed colors of their compounds.

Color of Compounds

The color of compounds, especially those containing transition metals, is a fascinating topic. These colors arise due to the absorption of specific wavelengths of light, leading to the dissipation of visible spectra as different colors. The color observed is usually the complementary color of the light absorbed.
In coordination complexes, the bonded ligands influence the splitting of d orbitals, which in turn affects the wavelengths of light absorbed. Here are some factors affecting the color of these compounds:

• The nature of the metal ion (e.g., its oxidation state and electronic configuration).
• Type and strength of the ligands attached to the metal.
• Geometry of the complex, which can alter the splitting pattern of the d orbitals.

For example, the cobalt complex \( ext{K}_3[\text{Co(NO}_2)_6] \) is yellow because of the specific arrangement of nitrate ligands, which influence electronic transitions within the cobalt center. Similarly, chromate ions in \( ext{BaCrO}_4 \) are responsible for the yellow hue through significant charge transfer actions.

Electronic Transitions

Electronic transitions refer to the movements of electrons between different energy levels within an atom or molecule. In transition metal complexes, these transitions often happen between d orbitals, known as d-d transitions, and are a major reason for their vibrant colors.
When light shines on a compound, electrons can absorb specific energy quanta, jumping from a lower energy d orbital to a higher one. Upon returning to their original state, they emit energy, manifesting as visible color.In coordination complexes, the d-d transitions are principally affected by:

• The crystal field splitting caused by the surrounding ligands.
• The specific metal center and its electron configuration.

These transitions vary significantly between different complexes. For instance, the absence of d-d transition in zinc complexes (due to fully filled d orbitals) results in colorless compounds like \(\text{Zn}_2[\text{Fe(CN)}_6]\), which, despite iron's presence, appears whitish due to insignificant electronic transitions.

Metal-Ligand Charge Transfer

Metal-Ligand Charge Transfer (MLCT) is a phenomenon that can contribute to the color of coordination complexes. This process involves an electron transfer from a metal's d orbital to a ligand's orbital or vice versa.
The MLCT is distinct from d-d transitions because these involve actual electron movement between the metal and ligands, rather than within the metal ion itself.Factors influencing MLCT include:

• The relative energies of the metal and ligand orbitals, which dictate the likelihood and extent of charge transfer.
• The nature of the metal center, with specific metals being more prone to MLCT transitions.
• The type of ligands, as some are more capable of accepting or donating electrons.

In the case of the discussed complexes, the yellow color in \(\text{BaCrO}_4\) arises partly due to extensive MLCT between the chromium metal and the oxide ligands, leading to a lower energy gap for visible light absorption. Meanwhile, the whitish nature of \(\text{Zn}_2[\text{Fe(CN)}_6]\) suggests minimal involvement of MLCT due to zinc's stable electronic configuration.