What will be the correct order for the wavelengths of absorption in the visible region for the following:
[Ni(NO2)6]4−, [Ni(NH3)6]2+, [Ni(H2O)6]2+
Solution 1
The central metal ion in all the three complexes is the same. Therefore, absorption in the visible region depends on the ligands. The order in which the CFSE values of the ligands increases in the spectrochemical series is as follows:
H2O < NH3 < NO2−
Thus, the amount of crystal-field splitting observed will be in the following order:`
`triangle_(@_((H_2O))) < triangle_(@_((NH_3))) < triangle_(@_((NO_2^-)))`
Hence, the wavelengths of absorption in the visible region will be in the order:
[Ni(H2O)6]2+ > [Ni(NH3)6]2+ > [Ni(NO2)6]4−
Solution 2
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: H20<NH3< NO2–; Thus, the energies absorbed for excitation will be in the order:
`[Ni(H_2O)_6]^(2+) < [Ni(NH_3)_6]^(2+) < [Ni(NO_2)_6]^(4-)`
As E = `(hc)/lambda` the wavelenghts absaorbed will be in the opposite order
