AIIMS 2005 Chemistry Spectrochemical Series MCQ Question

To determine the correct order for the wavelength of absorption in the visible region for the given coordination compounds, we need to understand the concept of the spectrochemical series and how it relates to the splitting of d-orbitals in transition metal complexes.

Understanding the Spectrochemical Series

1. Spectrochemical Series: The spectrochemical series is a list of ligands arranged according to the strength of the field they create around a central metal ion when forming a complex. Strong field ligands cause a larger splitting of the d-orbitals compared to weak field ligands. The general order is as follows (from strong to weak):

   • CN⁻ > CO > NH₃ > H₂O > OH⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻

2. Crystal Field Splitting: In an octahedral complex, the degeneracy of the d-orbitals is lifted due to the presence of ligands, leading to different energy levels. The energy difference between the two sets of d-orbitals (the higher energy $e_g$ level and the lower energy $t_{2g}$ level) is denoted as $\Delta$, which correlates with the wavelength of light absorbed: $\Delta = \frac{hc}{\lambda}$ where $h$ is Planck's constant, $c$ is the speed of light, and $\lambda$ is the wavelength of light absorbed.

Analysis of the Given Options

Now, let's evaluate the complexes provided in the options:

• [Ni(NH₃)₆]²⁺: Ammonia (NH₃) is a moderate field ligand.
• [Ni(H₂O)₆]²⁺: Water (H₂O) is a weaker field ligand compared to NH₃.
• [Ni(NO₂)₆]⁴⁻: Nitrite (NO₂⁻) is a relatively strong field ligand, particularly in the bidentate form.
• [Ni(NO₂)₄]²⁻: This is a weaker field ligand than [Ni(NO₂)₆]⁴⁻ but generally stronger than H₂O.

Correct Order for Wavelength of Absorption

• Option A:
  • [Ni(NO₂)₆]⁴⁻ < [Ni(NH₃)₆]²⁺ < [Ni(H₂O)₆]²⁺
  • This option posits that [Ni(NO₂)₆]⁴⁻ absorbs light at the longest wavelength (lowest energy), followed by [Ni(NH₃)₆]²⁺, and then [Ni(H₂O)₆]²⁺ at the shortest wavelength (highest energy). This is correct because the order of ligands indicates that NO₂⁻ would cause the least energy splitting, allowing for absorption of longer wavelengths, while H₂O causes greater splitting leading to shorter wavelengths.

Why Other Options are Incorrect

• Option B:

  • [Ni(NO₂)₄]²⁻ < [Ni(H₂O)₆]²⁺ < [Ni(NH₃)₆]²⁺
  • This suggests that [Ni(NO₂)₄]²⁻ absorbs at the longest wavelength, but it is incorrect as nitrite ligands in this form would not be strong enough to fit this order.

• Option C:

  • [Ni(H₂O)₆]²⁺ < [Ni(NH₃)₆]²⁺ < [Ni(NO₂)₄]²⁻
  • This incorrectly places [Ni(H₂O)₆]²⁺ as absorbing at a shorter wavelength than [Ni(NH₃)₆]²⁺, which goes against the known strength of the ligands.

• Option D:

  • [Ni(NH₃)₆]²⁺ < [Ni(NO₂)₄]²⁻ < [Ni(NO₂)₆]⁴⁻
  • It incorrectly suggests that [Ni(NH₃)₆]²⁺ absorbs at a shorter wavelength than both forms of NO₂, which is not consistent with the expected ligand strengths.

Conclusion

The correct order for the wavelength of absorption in the visible region is indeed:

A) [Ni(NO₂)₆]⁴⁻ < [Ni(NH₃)₆]²⁺ < [Ni(H₂O)₆]²⁺

This reflects the increasing