What will be the correct order for the wavelengths of absorption in the visible region for the following: [Ni(NO2)6]4-, [Ni(NH3)6]2+, [Ni(H2O)6]2+?
The correct order for wavelengths of absorption in the visible region is:
[Ni(NO₂)₆]⁴⁻ < [Ni(NH₃)₆]²⁺ < [Ni(H₂O)₆]²⁺
According to the spectrochemical series, the field strength of ligands increases in the order: H₂O < NH₃ < NO₂⁻. Stronger field ligands cause greater crystal field splitting energy (Δ₀). Since energy is inversely proportional to wavelength (E = hc/λ), a larger Δ₀ results in absorption of shorter wavelength light. Therefore, the complex with the strongest field ligand (NO₂⁻) absorbs at the shortest wavelength, while the complex with the weakest field ligand (H₂O) absorbs at the longest wavelength.
Explanation
This question tests understanding of the spectrochemical series and its relationship with crystal field splitting energy. The spectrochemical series arranges ligands in order of increasing field strength. Strong field ligands like NO₂⁻ cause larger splitting of d-orbitals (larger Δ₀), which corresponds to higher energy absorption. Since E = hc/λ, higher energy means shorter wavelength. The context in Q5.17-5.18 introduces these concepts, and the table shows examples of how different ligands affect absorption wavelengths in cobalt complexes. Students must apply this principle to nickel complexes.
Solution Steps
Step 1: Identify the field strength of each ligand from the spectrochemical series: H₂O (weak field) < NH₃ (moderate field) < NO₂⁻ (strong field).
Step 2: Relate field strength to crystal field splitting energy (Δ₀): Stronger field ligands produce larger Δ₀ values.
Step 3: Apply the inverse relationship between energy and wavelength (E = hc/λ): Larger Δ₀ means shorter wavelength absorbed.
Step 4: Arrange complexes in order of increasing wavelength: [Ni(NO₂)₆]⁴⁻ < [Ni(NH₃)₆]²⁺ < [Ni(H₂O)₆]²⁺.