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Question

Chemistry Question on coordination compounds

Consider the following complexes:
[CoCl(NH3)5]2+[CoCl(NH_3)_5]^{2+} (A)
[Co(CN)6]3[Co(CN)_6]^{3-} (B)
[Co(NH3)5(H2O)]3+[Co(NH_3)_5(H_2O)]^{3+} (C)
[Cu(H2O)4]2+[Cu(H_2O)_4]^{2+} (D)
The correct order of A, B, C, and D in terms of wavenumber of light absorbed is:

A

C < D < A < B

B

D < A < C < B

C

A < C < B < D

D

B < C < A < D

Answer

D < A < C < B

Explanation

Solution

As the ligand field strength increases, the energy of light absorbed by the complex also increases. Since wavenumber νˉ\bar{\nu} \propto energy of absorbed light, the order of wavenumber depends on the ligand strength.
For [Co(CN)6_6]3^{3-} (B), CN^- is a strong field ligand (highest νˉ\bar{\nu}).
For [Co(NH3_3)5_5(H2_2O)]3+^{3+} (C), NH3_3 and H2_2O have moderate ligand field strength.
For [Cu(H2_2O)4_4]2+^{2+} (D), H2_2O is a weaker field ligand.
For [CoCl(NH3_3)5_5]2+^{2+} (A), Cl^- is the weakest field ligand (lowest νˉ\bar{\nu}).
Thus, the order is:
D<A<C<B.D < A < C < B.