Question

What will be the correct order for the wavelength of absorption in the visible region of the following:

[Ni(NO₂)₆]^-4, [Ni(NH₃)₆]⁺², [Ni(H₂O)₆]⁺²

Answer 1

As the metal ion is fixed the increasing field strength (CFSE values) of the ligands from the spectrochemical series are in the order.

H₂O < NH₃ < NO₂^–

Thus the energies absorbed for excitation will be in the same order

[Ni(H₂O)₆]⁺² < [Ni(NH₃)₆]⁺² < [Ni(H₂O)₆]⁺²

As, E = hc/λ, the wavelength absorbed will be in the opposite order.