What will be the correct order for the wavelengths of absorption in the visible region for the following:
`[Ni(NO_(2))_(6)]^(4-), [Ni(NH_(3))_(6)]^(2+), [Ni(H_(2)O)_(6)]^(2+)` ?

Explanation

As metal ion is fixed the increasing field strengths i.e., the CFSE values of the ligands from the spectro chemical series are in the order `H_(2)O lt NH_(3) lt NO_(2)^(-)`
Thus, the energies absorbed for excitation will be in the order
`[Ni(H_(2)O)_(6)]^(2+) lt [Ni(NH_(3))_(6)]^(2+) lt [Ni(NO_(2))_(6)]^(4-)`
As `E=(hc)/lambda` the wavelengths absorbed will be in the opposite order.