Question
$[NiCl_{4}]^{2-}$ is paramagnetic while $[Ni(CO)_{4}]$ is diamagnetic though both are tetrahedral. Why?
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Answer
In $\left[ NiCl _4\right]^{2-}, Ni ^{2+}$ ion is coordinated with four $Cl ^{-}$ligands following $s p^3$ hybridisation with two unpaired electron on $Ni ^{2+}$, whereas in $\left[ Ni ( CO )_4\right]$, the Ni atom is in zero oxidation state. In the presence of strong ligand effect of CO , the unpaired electrons of Ni pair up and coordinated with four CO ligands following $s p^3$ hybridisation, having no unpaired electron on Ni atom, $Cl ^{-}$being a weak ligand is unable to pair up the unpaired electrons of $Ni ^{2+}$.
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