JEE Main 2025 — Coordination Compounds Question with Solution

To determine which molecules or species are paramagnetic, we need to identify those with unpaired electrons. Here's the analysis:

${\mathrm{O}}_2$: This molecule has 2 unpaired electrons as predicted by the Molecular Orbital Theory (MOT), making it paramagnetic.

${\mathrm{O}}_2^{+}$: This ion has 1 unpaired electron according to MOT, which also makes it paramagnetic.

${\mathrm{O}}_2^{-}$: This ion contains 1 unpaired electron as determined by MOT, indicating it is paramagnetic.

$\mathrm{NO}$: As a species with an odd number of electrons, $\mathrm{NO}$ has unpaired electrons, making it paramagnetic.

${\mathrm{NO}}_2$: This molecule is also an odd-electron species, which means it has unpaired electrons and is paramagnetic.

${\mathrm{K}}_2\left[{\mathrm{NiCl}}_4\right]$: In this compound, ${\mathrm{Ni}}^{2+}$ (nickel with a +2 oxidation state) has an electron configuration of $3d^8$. Given the presence of weak field ligands and a coordination number of 4, this forms a tetrahedral complex. As a result, it is paramagnetic with 2 unpaired electrons.

Thus, the paramagnetic species in the provided list are ${\mathrm{O}}_2$, ${\mathrm{O}}_2^{+}$, ${\mathrm{O}}_2^{-}$, $\mathrm{NO}$, ${\mathrm{NO}}_2$, and ${\mathrm{K}}_2\left[{\mathrm{NiCl}}_4\right]$.