# Which among the following is a paramagnetic complex? (At No. of Mo=42, Pt=78) Option 1) $Mo(CO)_{6}$ Option 2) $\left [ Co(NH_{3})_{6} \right ]^{3}$ Option 3) $\left [ Pt(en)Cl_{2} \right ]$ Option 4) $\left [ CoBr_{4} \right ]^{2-}$

P Plabita

As we learnt in

Magnetic Moments (spin only) -

$\sqrt{n\left ( n+2 \right )}$ where n= number of unpaired electron.

- wherein

Number of unpaired $e^{-} s$ and corresponding magnetic moments

$\\1\rightarrow 1.7\\2\rightarrow 2.8\\3\rightarrow 3.9\\4\rightarrow 4.9\\5\rightarrow 5.9$

Since $Br^{-}$  is a weak feild ligand, so electrons in $Co^{2+}$ will not pair up.

$\therefore$ the complex has 3 unpaired electrons leaving it paramagnetic in nature.

Option 1)

$Mo(CO)_{6}$

This is incorrect option

Option 2)

$\left [ Co(NH_{3})_{6} \right ]^{3}$

This is incorrect option

Option 3)

$\left [ Pt(en)Cl_{2} \right ]$

This is incorrect option

Option 4)

$\left [ CoBr_{4} \right ]^{2-}$

This is correct option

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