JEE Chemistry 2026 Question with Solution

Given: Two statements about magnetic properties of coordination compounds must be checked.

Find: Which option correctly identifies the truth values of Statement I and Statement II.

For each complex, determine the oxidation state, write the $d$-electron configuration, use ligand strength and geometry, and then count unpaired electrons.

Statement I

• $[Cu(NH_3)_4]^{2+}$: Copper is $+2$, so $Cu^{2+} = 3d^9$. A $d^9$ system has 1 unpaired electron.
• $[Ni(en)_3]^{2+}$: Nickel is $+2$, so $Ni^{2+} = 3d^8$. Octahedral complex with configuration $t_{2g}^6 e_g^2$, so it has 2 unpaired electrons.
• $[Ni(NH_3)_6]^{2+}$: Nickel is $+2$, so $Ni^{2+} = 3d^8$. Octahedral complex with configuration $t_{2g}^6 e_g^2$, so it also has 2 unpaired electrons.
• $[Mn(H_2O)_6]^{2+}$: Manganese is $+2$, so $Mn^{2+} = 3d^5$. With weak-field $H_2O$ in an octahedral high-spin complex, configuration is $t_{2g}^3 e_g^2$, giving 5 unpaired electrons.

So the numbers of unpaired electrons are $1, 2, 2,$ and $5$ respectively. Therefore, $[Mn(H_2O)_6]^{2+}$ has the maximum number of unpaired electrons.

Statement I is true.

Statement II

Now examine the species in the given pairs.

• $[NiCl_4]^{2-}$: Nickel is $+2$, so $3d^8$. Since $Cl^-$ is a weak-field ligand, the complex is tetrahedral and has 2 unpaired electrons. Hence it is paramagnetic.
• $[Ni(CO)_4]$: Nickel is in oxidation state $0$. With strong-field $CO$, the metal attains paired electrons and the complex is diamagnetic.
• $[Ni(CN)_4]^{2-}$: Nickel is $+2$, so $3d^8$. Since $CN^-$ is a strong-field ligand, the complex is square planar and diamagnetic.

Check the pairs:

1. $\{[NiCl_4]^{2-}, [Ni(CO)_4]\}$: one paramagnetic and one diamagnetic, so not only diamagnetic.
2. $\{[NiCl_4]^{2-}, [Ni(CN)_4]^{2-}\}$: one paramagnetic and one diamagnetic, so not only diamagnetic.
3. $\{[Ni(CO)_4], [Ni(CN)_4]^{2-}\}$: both are diamagnetic.

Thus, the number of pairs containing only diamagnetic species is one, not two.

Statement II is false.

Therefore, Statement I is true but Statement II is false.

The correct option is B.