Question

Correct increasing order for the wavelengths of absorption in the visible region by the complexes of `Co^(3+)` is:
A. `[Co(en)_(3)]^(3+),[Co(NH_(3))_(6)]^(3+),[Co(H_(2)O)_(6)]^(3+)`
B. `[Co(H_(2)O)_(6)]^(3+),[Co(en)_(6)]^(3+),[Co(NH_(3))_(6)]^(3+)`
C. `[Co(H_(2)O)_(6)]^(3+),[Co(NH_(3))_(6)]^(3+),[Co(en)_(3)]^(3+)`
D. `[Co(NH_(3))_(6)]^(3+),[Co(en)_(3)]^(3+),[Co(H_(2)O)_(6)]^(3+)`

Answer 1

Correct Answer - A
(a) Key concept Wavelength (`lamda`) of absorption is mversely proportional to CFSE (`Delta_(O)` value) of ligands attached with the central metal ion
i.e., `lamdaprop1/(Delta_(O))`
According to spectrochemical series.
`I^(-) lt Br^(-) lt S^(2-) lt SCN^(-) lt Cl^(-) lt F^(-) lt OH^(-)`
`lt C_(2)O_(4)^(2-)ltO^(2-) lt H_(2)O lt NSS^(-) lt NH_(3) lt en lt NO_(2)^(-)ltCN^(-)`

Toe CFSE of ligands attached with `Co^(3+)` ion is in the order.
`en gt NH_(3) gt H_(2)O` (From spectrochemical series)
`therefore` Wavelength of absorbed light `(lamda)prop1/(Delta_(O))`
`therefore` For ligand the order of wavelength of absorption in the visible region will be `en lt NH_(3) lt H_(2)O`
or `[Co(en)_(3)]^(3+)lt[Co(NH_(3))_(6)]^(3+)lt[Co(H_(2)O)_(6)]^(3+)`