Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds is violet in colour? (a) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN}), \mathrm{NSO}_{3}\right]_{3}\) (b) \(\mathrm{Na}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6} \mathrm{NSO}\right]_{2}\) (c) \(\mathrm{Na}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NSO}\right]_{3}\) (d) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]\)

Step-by-Step Solution

Answer

Option (c), \( \mathrm{Na}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{5}\mathrm{NSO}\right]_{3} \), is violet.

Step 1: Understand the Coordination Complexes

To determine which compound is violet, we need to analyze the given coordination complexes. Coordination complexes are composed of a central metal atom (Fe in this case) and surrounding molecules or ions (ligands) such as \( \text{CN}^- \) (cyanide) and \( \text{NSO}^- \) (thionitrosyl). The arrangement or nature of these molecules affects the color.

Step 2: Identify the Common Indicator of Color

Color in coordination compounds is often due to the electronic transitions between d-orbitals (d-d transitions) of the metal center. The specific ligands attached influence these energy gaps and hence the colors observed.

Step 3: Examine the Ligands Present

Each complex contains iron (Fe) and varying numbers of cyanide (CN) and \( \text{NSO}^- \) groups. The \( \text{NSO}^- \) ligand, when coordinated in a particular way with Fe, tends to give the complex a violet or purple color.

Step 4: Evaluate the Color Based on Chemical Knowledge

Historically, it is known that the complex with the formula \( \text{Na}_2\left[\text{Fe}(\text{CN})_5 \text{NOS}\right]_3 \) is characterized by a violet color due to the specific coordination of the ligands around the iron center, which is option (c).

Step 5: Interpret the Result

Given the choices, only \( \text{Na}_2\left[\text{Fe}(\text{CN})_5 \text{NOS}\right]_3 \) is known to show a violet color based on the arrangement and interaction of its ligands.

Key Concepts

Ligands

Ligands are molecules or ions that bind to a central metal atom in a coordination complex. They act like "attachments" around the metal core. In coordination compounds, ligands significantly affect the chemical properties and stability of the complex.

• Types of ligands: Ligands can be classified based on the number of donor atoms. These can be monodentate (one donor atom, like \({\mathrm{CN}}^-\)), bidentate (two bonding sites), or polydentate (multiple bonding sites).
• Role and function: Ligands influence the electronic environment of the metal atom. They alter the energy levels of the d-orbitals (explained later), affecting properties like reactivity and color of the complex.

Ligands are crucial in determining the geometry and color of the coordination complex. For example, the presence and coordination of cyanide (\({\mathrm{CN}}^-\)) and nitrosyl units (\({\mathrm{NSO}}^-\)) with iron in coordination compounds can lead to a vivid display of colors, helping us identify these compounds based on visual cues.

Electronic Transitions

Electronic transitions are movements of electrons between energy levels within an atom or molecule. In transition metal complexes, this specifically refers to the changes in electron positions among the d-orbitals of the metal atom.

• Transition types: When electrons absorb energy, they transition from a lower energy level to a higher one, often between d-orbitals. This absorption of energy leads to various phenomena, including color changes in coordination compounds.
• Influence of ligands: The type of ligands around a metal can alter these electronic transitions. Strong field ligands, such as cyanide, create larger energy gaps between d-orbitals than weak field ligands, changing the energy required for transitions.

Electronic transitions are the crux of why many coordination compounds are colored, as light is absorbed at energies corresponding to visible light wavelengths, leading to the color observed in these substances.

d-d Transitions

d-d transitions are a specific type of electronic transition occurring within d-orbitals of transition metal ions. They're central to explaining the colors observed in many coordination compounds.

• Nature of d-d transitions: These occur when an electron jumps from one d-orbital to another, within the same atom, upon absorbing light energy.
• Influence of ligands and geometry: The exact energy required for these transitions is influenced by the geometry of the complex and the types of ligands present. An octahedral complex, for instance, will have d-d transitions at different energies than a tetrahedral one.

These transitions don't allow large amounts of light energy, hence are often responsible for the typical pastel colors in transition metal complexes. The strength of the ligand field impacts transition energy and intensity, explaining why different ligands lead to different solution colors.

Color in Coordination Compounds

The vibrant colors of many coordination compounds are direct results of electronic interactions within the complexes. They are influenced predominantly by d-d transitions and the nature of the ligands surrounding the metal.

• Color generation mechanism: When light hits a coordination complex, certain wavelengths of light are absorbed for d-d transitions. The light not absorbed is reflected, and that's the color we perceiv (e.g., if a compound absorbs all but red light, it will appear red).
• Role of ligand types: As seen in the violet color of the compound \(\mathrm{Na}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]_{3}\), the specific ligands and their arrangement play a crucial role in shaping color due to how they affect the metal's electronic environment.

Understanding color in coordination complexes involves appreciating the complex interplays of chemistry. Ligands, metal types, and their spatial arrangement all intertwine to paint the rich hues that signify unique chemical structures and properties.