Question

Arrange the following in order of decreasing number of unpaired electrons?
I. $\left[ Fe \left( H _{2} O \right)_{6}\right]^{2+}$
II. $\left[ Fe ( CN )_{6}\right]^{3-}$
III. $\left[ Fe ( CN )_{6}\right]^{4-}$
IV. $\left[ Fe \left( H _{2} O \right)_{6}\right]^{3+}$

• A. IV, I, II, III
• B. I, II, III, IV
• C. III, II, I, IV
• D. II, III, I, IV

Answer 1

Answer: (A)

(i) $\left[ Fe \left( H _{2} O \right)_{6}\right]^{2+}, 3 d ^{6}$



$n = 4$ (weak field ligand)

(ii) $\left[ Fe ( CN )_{6}\right]^{3-}, 3d ^{5}$



$n = 1$ (Strong field ligand)

(iii) $\left[ Fe ( CN )_{6}\right]^{4-}, 3d ^{6}$



$n = 0$ (strong field ligand)

(iv) $\left[ Fe \left( H _{2} O \right)_{6}\right]^{3-}, 3 d ^{5}$



$n = 5$ (weak field ligand)