Question

Which of the following cyano complexes would exhibit the lowest value of paramagnetic bshaviour, ?

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

Answer 1

Answer: (D)

Outer electronic configuration of $M^{3+}$ ions are :
$C{r}^{3+}:3d^3,M{n}^{3+}:3d^4,F{e}^{3+}:3d^5$ and $C{o}^{3+}:3d^6$ ligand has a tendency to pair up $AsC{N}^{-}(n-1)d$ electrons for $d^{2}s{p}^3$ hybridisation number of unpaired electrons in these complexes will be
$[Cr(CN{)}_6{]}^{3-}$ 3 unpaired electrons (No pairing up)
$[Mn(CN{)}_6{]}^{3-}$ 2 unpaired electrons
$[Fe(CN{)}_6{]}^{3-}$ 1 unpaired electron
$[Co(CN{)}_6{]}^{3-}$ No unpaired electro
Therefore, complex expected to have lowest paramagnetic behaviour (with no unpaired electron) is $[Co(CN{)}_6{]}^{3-}$