Question

Which one of the following cyano complexes would exhibit the lowest value of paramagnetic behaviour?

(Atomic Numbers: $Cr=24;Mn=25;Fe=26;Co=27$)

• A. $\left[Fe\right(CN{)}_6{]}^{3-}$
• B. $\left[Co\right(CN{)}_6{]}^{3-}$
• C. $\left[Cr\right(CN{)}_6{]}^{3-}$
• D. $\left[Mn\right(CN{)}_6{]}^{3-}$

Answer 1

The correct option is B $\left[Co\right(CN{)}_6{]}^{3-}$

$CN$ is a strong field ligand, all complexes will be inner orbital i.e $d^{2}s{p}^3$

a) $\left[Fe\right(CN{)}_6{]}^{3-}$ $F{e}^{3+}$ Configuration $\left[Ar\right]3d^5$

$\upharpoonleft \upharpoonright \upharpoonleft \upharpoonright \upharpoonleft \square \square$

$1$ unpaired electron.

$u=\sqrt{3}BM$

b) $\left[Co\right(CN{)}_6{]}^{3-}$ $C{o}^{3+}$ Configuration $\left[Ar\right]3d^6$

$\upharpoonleft \upharpoonright \upharpoonleft \upharpoonright \upharpoonleft \upharpoonright \square \square$

$0$ unpaired electron

$u=0BM$

c) $\left[Cr\right(CN{)}_6{]}^{3-}$ $C{r}^{+3}$ Configuration $\left[Ar\right]3d^3$

$\upharpoonright \upharpoonright \upharpoonright \square \square$

3 unpaired electrons

$u=\sqrt{n(n+2)}$

$n=$ number of electrons

$u=\sqrt{15}BM$

d) $\left[Mn\right(CN{)}_6{]}^{3-}$ Configuration $\left[Ar\right]3d^4$

$\upharpoonleft \upharpoonright \upharpoonright \upharpoonleft \square \square$

$2$ unpaired electrons.

$u=\sqrt{2\times 4}=\sqrt{8}BM$

Therefore, $C{o}^{+3}$ exhibits lowest paramagnetic behavior with strong field ligand.