Question

Three complexes,

[CoCl(NH₃)₅]²⁺ (I)

[Co(NH₃)₅H₂O]³⁺ (II) and

[Co(NH₃)₆]³⁺ (III)

absorb light in the visible region. The correct order of the wavelength of light absorbed by them is:
1. (III) > (I) > (II)
2. (III) > (II) > (I)
3. (II) > (I) > (III)
4. (I) > (II) > (III)

Answer 1

Correct Answer - Option 4 : (I) > (II) > (III)

Concept:

Wavelength of the energy absorbed by the coordination compound is inversely proportional to crystal field stabilization energy (CFSE).

CFSE depends on four main properties:

(i) Geometry like octahedral or tetrahedral.

(ii) Charge of the compound in case of metals.

(iii) Nature of ligand as like strong or weak.

(iv) Size of the orbital like 3d, 4d, or 5d etc.

From the question, the three compounds are of Cobalt thus all three possess same octahedral structure and geometry does not work here.

Now all the three cobalt must have charge of 3+ and we cannot classify according to charge.

Similarly size also does not play any differentiating role in here. So only nature of the ligand varies.

Wavelength of the energy absorbed by the coordination compound is inversely proportional to ligand field strength of the given coordination compound. The strong field ligand can cause higher splitting of the d-orbitals. The decreasing order of the ligand field strength is NH₃ > H₂O > Cl.

Therefore, decreasing order of the absorbed wavelength is

(I) > (II) > (III)

So, option (d) is the correct answer.