MCQ
Which of the following complexes exhibits the highest paramagnetic behaviour ?

where $gly =$ glycine, $en =$ ethylenediamine and  $bpy =$ bipyridyl moities. (At. nos. $Ti = 22, V = 23,$$ Fe = 26, Co = 27$)

  • A
    $[Co(ox)_2(OH)_2]^-$
  • B
    $[Ti(NH_3)_6]^{3+}$
  • C
    $[V(gly)_2(OH)_2(NH_3)_2]^+$
  • $[Fe(en)(bpy)(NH_3)_2]^{2+}$

Answer

Correct option: D.
$[Fe(en)(bpy)(NH_3)_2]^{2+}$
d
Oxidation State of $Ti$ in the complex $\left[\mathrm{Ti}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$

$\mathrm{Ti}^{3+}=1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{1}$

no. of unpaired electron in $d$ orbital is one. Oxidation State of $V$ in complex $\left[\mathrm{V}(\mathrm{gly})_{2}(\mathrm{OH})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{+}$

$=x+2 \times 0+2 \times(-1)+2 \times 0=+1$

$\therefore x=+3$

$\mathrm{V}^{3+}=1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{2}$

no. of unpaired electron in $d$ orbital is two.

Oxidation State of $Fe$ in the given complex is $+2$

$\therefore \quad \mathrm{Fe}^{2+}=1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{6}$

no. of unpaired electron in $d$ orbital is four.

Oxidation State of $\mathrm{Co}$ in the given complex $\left[\mathrm{Co}(ox)_{2}(\mathrm{OH})_{2}\right]^{-}$

$=x+2 \times(-2)+2 \times(-1)=-1=x-4-2=-1$

$\therefore x=+5$

(not possible, common ox. no. of $\mathrm{Co}=+2,+3,+4$ ) $\mathrm{Co}^{5+}=1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{4}$

It should be an inner orbital complex ($ d'sp$ hybridisation) containing only one unpaired electron. So the complex having highest paramagnetism would be the complex of iron containing four unpaired electrons.

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