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Q5.31:
What will be the correct order for the wavelengths of absorption in the visible region for the following: ([Ni(NO_{2})_{6}]^{4-}), ([Ni(NH_{3})_{6}]^{2+}), ([Ni(H_{2}O)_{6}]^{2+})?

Solution :

Step 1: Theoretical Basis of Crystal Field Theory (CFT)

The wavelength of light absorbed ($\lambda$) by a coordination complex in the visible region is inversely proportional to the crystal field splitting energy ($\Delta_o$). This relationship is governed by the Planck-Einstein relation:

$\Delta_o = E = \dfrac{hc}{\lambda}$

Where:

  • $h$ is Planck's constant.
  • $c$ is the speed of light.
  • $\Delta_o$ is the energy gap between $t_{2g}$ and $e_g$ orbitals.

[Per the Crystal Field Theory, the magnitude of $\Delta_o$ depends on the nature of the ligand according to the Spectrochemical Series]. As the field strength of the ligand increases, $\Delta_o$ increases, which results in a decrease in the wavelength ($\lambda$) of absorbed light.

Step 2: Analysis of Ligand Field Strength

The ligands present in the given complexes are $NO_2^-$, $NH_3$, and $H_2O$. Based on the established Spectrochemical Series, the order of their field strengths is:

$H_2O < NH_3 < NO_2^-$

Consequently, the crystal field splitting energy ($\Delta_o$) follows the same order:

$\Delta_o([Ni(H_2O)_6]^{2+}) < \Delta_o([Ni(NH_3)_6]^{2+}) < \Delta_o([Ni(NO_2)_6]^{4-})$

Step 3: Deriving the Inverse Wavelength Order

Given that $\lambda \propto \dfrac{1}{\Delta_o}$, the order of wavelengths for the absorption of light is the exact inverse of the crystal field splitting energy:

$\lambda([Ni(NO_2)_6]^{4-}) < \lambda([Ni(NH_3)_6]^{2+}) < \lambda([Ni(H_2O)_6]^{2+})$

Step 4: Visualizing the Energy Correlation

Ligand Field Strength (Increasing) H₂O NH₃ NO₂⁻ Δₒ increases

Summary Comparison Table

Complex Ligand Field Strength $\Delta_o$ Magnitude Wavelength ($\lambda$)
$[Ni(H_2O)_6]^{2+}$ Weakest Lowest Highest
$[Ni(NH_3)_6]^{2+}$ Intermediate Intermediate Intermediate
$[Ni(NO_2)_6]^{4-}$ Strongest Highest Lowest

Final Solution: The correct order for the wavelengths of absorption is: $[Ni(NO_2)_6]^{4-} < [Ni(NH_3)_6]^{2+} < [Ni(H_2O)_6]^{2+}$.


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