Question

Which one of the following species attracts to an external magnetic field ?

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Answer 1

$C{N}^{-}\text{and}CO$ are strong field ligands and causes pairing of electrons. Whereas $H_{2}O$ is weak field ligand so do not cause pairing, generally.
$\left[Fe\right(H_{2}O{)}_6{]}^{3+}$


So, $\left[Fe\right(H_{2}O{)}_6{]}^{3+}$ is paramagnetic. In the rest of the complexes no unpaired electron is present hence they are diamagnetic. Paramagnetic substances get weakly attracted by external magnetic field whereas diamagnetic gets weakly repelled.

$\left[Ni\right(CN{)}_4{]}^{2-}\to 3d^8\text{(All electrons are paired})$
$\left[Co\right(CN{)}_6{]}^{3-}\to 3d^6\text{(All electrons are paired})$
$\left[Ni\right(CO{)}_4]\to 3d^8\text{(All electrons are paired})$

Hence, option (b) is correct.