Question

The colour of the coordination compounds depends upon the crystal field splitting. What will be the correct order of absorption of wavelength of light in the visible region for the complexes $\underset{(I)}{{[Co{(N{H}_3)}_6]}^{3+},}$ $\underset{(II)}{{[Co{(CN)}_6]}^{3-},}$ $\underset{(III)}{{[Co{(H_{2}O)}_6]}^{3+}?}$

• A. II > I > III
• B. I > III > II
• C. III > I > II
• D. II > I > III

Answer 1

Answer: (C)

Different complexes exhibit different colours when either the metal ion is different or the metal ion is same but ligands attached to it are different. The CFSE of the ligands. is in the order $H_{2}O<N{H}_3<C{N}^{-}$ (from spectrochemical series). The wavelengths absorbed will be in the opposite order $\left(\because E=\frac{hc}{\lambda }\right)$ i.e., lll > I > II The complementary colour will be transmitted or reflected.