Question

In which of the following, complex ions are not in correct order with respect to their magnitude of crystal field splitting?

• A. [Fe(H$_2$O)$_6$]$^{3+}$ > [FeF$_6$]$^{3-}$
• B. [Fe(en)$_3$]$^{3+}$ > [Fe(NCS)$_6$]$^{3-}$
• C. [Fe(CN)$_6$]$^{4-}$ > [Fe(H$_2$O)$_6$]$^{2+}$
• D. [Fe(H$_2$O)$_6$]$^{2+}$ > [Fe(NH$_3$)$_6$]$^{2+}$

Hint:

Remember the spectrochemical series: CN$^-$ > en > NH$_3$ > H$_2$O > F$^-$ > OH$^-$ > Cl$^-$ — stronger ligands cause greater crystal field splitting.

Answer 1

The Correct Option is D

Step 1: Understand Crystal Field Splitting. 
Crystal field splitting depends on the ligand field strength. Ligands are arranged in the spectrochemical series. 
Step 2: Compare the ligands involved. 
Ammonia (NH$_3$) is a stronger field ligand than water (H$_2$O), so it causes more splitting. 
Step 3: Analyze the given order. 
The statement “[Fe(H$_2$O)$_6$]$^{2+}$ > [Fe(NH$_3$)$_6$]$^{2+}$” is incorrect because H$_2$O is weaker than NH$_3$.