Question

The $d$ electron configuration of $Cr ^{2+}, Mn ^{2+}, Fe ^{2+}$ and $Ni ^{2+}$ are $3 d ^{4}, 3 d ^{5}, 3 d ^{6}$ and $3 d ^{8}$ respectively. Which one of the following aqua complex will exhibit the minimum paramagnetic behaviour?
(At. Nos. $Cr =24, Mn =25, Fe =26, Co =27$ )

• A. $[Fe(H_2O)_6]^{2+}$
• B. $\left[ Ni \left( H _{2} O \right)_{6}\right]^{2+}$
• C. $[Co(H_2O)_6]^{2+}$
• D. $[Mn(H_2O)_6]^{2+}$

Answer 1

Answer: (B)

As the number of unpaired electron increases, the magnetic moment increases and hence, the paramagnetic behaviour increases.
So, $Cr ^{2+}(22)=3 d^{4}$ (4 unpaired electrons)
$Mn ^{2+}(23)=3 d^{5}$(5 unpaired electrons)
$Fe ^{2+}(24)=3 d^{6}$(4 unpaired electrons)
$Ni ^{2+}(26)=3 d^{8}$(2 unpaired electrons)
So, $\left[ Ni \left( H _{2} O \right)_{6}\right]^{2+}$ exhibit minimum paramagnetic behaviour.