Question

Consider the following complexes:
\([CoCl(NH_3)_5]^{2+}\) (A)
\([Co(CN)_6]^{3-}\) (B)
\([Co(NH_3)_5(H_2O)]^{3+}\) (C)
\([Cu(H_2O)_4]^{2+}\) (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 1

The Correct Option is B

As the ligand field strength increases, the energy of light absorbed by the complex also increases. Since wavenumber $\bar{\nu} \propto$ energy of absorbed light, the order of wavenumber depends on the ligand strength.
For [Co(CN)$_6$]$^{3-}$ (B), CN$^-$ is a strong field ligand (highest $\bar{\nu}$).
For [Co(NH$_3$)$_5$(H$_2$O)]$^{3+}$ (C), NH$_3$ and H$_2$O have moderate ligand field strength.
For [Cu(H$_2$O)$_4$]$^{2+}$ (D), H$_2$O is a weaker field ligand.
For [CoCl(NH$_3$)$_5$]$^{2+}$ (A), Cl$^-$ is the weakest field ligand (lowest $\bar{\nu}$).
Thus, the order is:
\[D < A < C < B.\]