The colour of the coordination compounds depends on the crystal field splitting . What will be the correct order of absorption of wavelength of light in the visible region for the complexes , `[Co(NH_(3))_(6)]^(3+) , [Co(CN)_(6)]^(3-) , [Co(H_(2)O)_(6)]^(3+)` ?

Options

A. `[Co(CN)_6]^(3-) gt [Co(NH_3)_6]^(3+) gt [Co(H_2O)_6]^(3+)`
B. `[Co(NH_3)_6]^(3+) gt [Co(H_2O)_6]^(3+) gt [Co(CN)_6]^(3-)`
C. `[Co(H_2O)_6]^(3+) gt [Co(H_2O)_6 ]^(3+) gt [Co(CN)_6]^(3-)`
D. `[Co(CN)_6]^(3-) gt [Co(H_2O)_6]^(3+) gt [Co(CN)_6]^(3+)`

Answer

C

Explanation

The CFSE of the ligands is in the order : `[Co(H_2O)_6]^(3+) lt [Co(NH_3)_6]^(3+) lt [Co(CN)_6]^(3-)`
Hence, excitation energies will be in the order : `[Co(H_2O)_6]^(3+) lt [Co(NH_3)_6]^(3+) lt [Co(CN)_6]^(3-)`
and the absorption of wavelength of light will in the reverse order, i.e.
`underset(("Orange"))([Co(H_2O)_6]^(3+)) gt underset(("Blue"))([Co(NH_3)_6]^(3+)) gt underset(("Violet"))([Co(CN)_6]^(3-))`