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 (CFSE values) of the ligands from the spectrochemical series are in the order : `H_(2)OltNH_(3)ltNO_(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 wavelength absorbed will be in the opposite order.