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

Options

A. `[Co(CN)_(6)]^(3-) gt [Co(NH_(3))_(6)]^(3+) gt [Co(H_(2)O)_(6)]^(3+)`
B. `[Co(NH_(3))_(6)]^(3+) gt [Co(H_(2)O)_(6)]^(3+) gt [Co(CN)_(6)]^(3-)`
C. `[Co(H_(2)O)_(6)]^(3+) gt [Co(NH_(3))_(6)]^(3+) gt [Co(CN)_(6)]^(3-)`
D. `[Co(CN)_(6)]^(3-) gt [Co(NH_(3))_(6)]^(3+) gt [Co(H_(2)O)_(6)]^(3+)`

Answer

C

Explanation

As we know that, strong field ligand split the five degenerate energy levels with more energy separation than weak field ligand, i.e., as strength of ligand increases crystal field splitting energy increases.
Hence, `DeltaE=(hc)/lambda`
`rArr " "DeltaE prop 1/lambda rArr lambda prop 1/(Delta E)`
As energy separation increases, the wavelength decreases.
Thus, the correct order is
`[Co(H_(2)O)_(6)]^(3+) gt [Co(NH_(3))_(6)]^(3+) gt [Co(CN)_(6)]^(3-)`
Here, strength of ligand increases, `DeltaE` increases, `CFSE` increases and `lambda` absored decreases.
Hence, correct choice is (c).