Consider two complexes: (I) [Fe(CN)₆]⁴^-, and (II) [Fe(H₂O)₆]²⁺. In view of the spectrochemical series, which one of the following sets of predictions is most likely correct? Spectrochemical Series I^-

Question

Consider two complexes: (I) [Fe(CN)₆]⁴^-, and (II) [Fe(H₂O)₆]²⁺. In view of the spectrochemical series, which one of the following sets of predictions is most likely correct? Spectrochemical Series I^- < Br ^- < Cl^- < F^- < OH^- < H₂O < NH₃ < NO₂^- < CN^- < CO

Quizplus · Accepted Answer

According to the spectrochemical series, the ligand CN- has strong field and causes the crystal field splitting to be large. So, it is expected that complex I with CN- ligand would have a high CFSE and the electrons would be stabilized in the lower energy t2g orbitals. Therefore, complex I is expected to be low spin.
On the other hand, ligand HS- has a weak field and causes a small CFSE. So, complex II with HS- ligand is expected to have a low CFSE and the electrons would be promoted to higher energy eg orbitals. Therefore, complex II is expected to be high spin.
Also, complex I has a Fe(II) center while complex II has a Fe(III) center. Fe(II) has 6 d electrons and can go both high or low spin depending on the ligand environment. Similarly, Fe(III) also have 6 d electrons and can also go either high or low spin. However, based on the ligand environment as discussed above, complex I is expected to be low spin and complex II is expected to be high spin.
Lastly, since complex II has a weaker ligand HS- than CN- in complex I, it is expected to absorb light at the longer wavelength. Therefore, option B is the correct choice.

Consider Two Complexes: (I) [Fe(CN)6]4-, and (II) [Fe(H2O)6]2+

Consider Two Complexes: (I) [Fe(CN)₆]⁴^-, and (II) [Fe(H₂O)₆]²⁺