Question

The correct order of the complexes
[5pt] $[\text{Co(NH}_3)_5(\text{H}_2\text{O})]^{3+}$ (A),
[3pt] $[\text{Co(NH}_3)_6]^{3+}$ (B),
[3pt] $[\text{Co(CN)}_6]^{3-}$ (C),
[3pt] $[\text{CoCl(NH}_3)_5]^{2+}$ (D)
[5pt] in terms of wavelength of light absorbed is:

• A. D $>$ A $>$ B $>$ C
• B. C $>$ B $>$ D $>$ A
• C. D $>$ C $>$ B $>$ A
• D. C $>$ B $>$ A $>$ D

Hint:

Remember the spectrochemical series and how it relates to the strength of ligands and the energy of absorbed light.

Answer 1

The Correct Option is A

To determine the correct order of the complexes in terms of the wavelength of light absorbed, we need to consider the crystal field splitting energy (\( \Delta \)) and the nature of ligands in each complex.

In the context of crystal field theory, the strength of the ligand affects the splitting of the d-orbitals in a transition metal complex. The spectrochemical series helps us in identifying ligand strength. The series approximately is:

• Strong field ligands: CN^- > NO₂^- > en > NH₃
• Weak field ligands: H₂O > OH^- > F^- > Cl^-

The more the field strength of the ligand, the larger the splitting, and vice versa.

Now, let us analyze each complex:

1. Complex D: [CoCl(NH₃)₅]²⁺
   • Ligands: NH₃ (stronger than Cl^-), Cl^- (weak field ligand)
   • Weakest ligand mixture, thus smallest splitting and therefore absorbs the longest wavelength.
2. Complex A: [Co(NH₃)₅(H₂O)]³⁺
   • Ligands: NH₃, H₂O (both relatively stronger than Cl^-)
   • Absorbs shorter wavelength than D.
3. Complex B: [Co(NH₃)₆]³⁺
   • Ligands: NH₃ (all are NH₃, strong field ligand)
   • Stronger splitting than A, thus absorbs even shorter wavelength.
4. Complex C: [Co(CN)₆]^3-
   • Ligands: CN^- (very strong field ligand)
   • Strongest field splitting, absorbs the shortest wavelength.

Thus, the order from longest to shortest wavelength is:

D > A > B > C

This matches the given option: D $>$ A $>$ B $>$ C

Answer 2

\[ \text{We know } E = h\nu = \frac{hC}{\lambda} \] \[ E \propto \frac{1}{\lambda} \] Here all Co in +3 oxidation state. So, as the ligand field strength ↑, CFSE ↑ Order of field strength of ligand : \[ CN^->NH_3>H_2O>Cl^- \] CFSE order : C $>$ B $>$ A $>$ D Wavelength order : D $>$ A $>$ B $>$ C