Given below are two statements :

**Statement I :** Among $[\mathrm{Cu(NH_3)_4}]^{2+}$, $[\mathrm{Ni(en)_3}]^{2+}$, $[\mathrm{Ni(NH_3)_6}]^{2+}$ and $[\mathrm{Mn(H_2O)_6}]^{2+}$, $[\mathrm{Mn(H_2O)_6}]^{2+}$ has the maximum number of unpaired electrons.

**Statement II :** The number of pairs among ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CO)_4}]}$, ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CN)_4}]^{2-}}$ and ${[\mathrm{Ni(CO)_4}],[\mathrm{Ni(CN)_4}]^{2-}}$ that contain only diamagnetic species is two.

In the light of the above statements, choose the correct answer from the options given below :

Question

Given below are two statements :

**Statement I :** Among $[\mathrm{Cu(NH_3)_4}]^{2+}$, $[\mathrm{Ni(en)_3}]^{2+}$, $[\mathrm{Ni(NH_3)_6}]^{2+}$ and $[\mathrm{Mn(H_2O)_6}]^{2+}$, $[\mathrm{Mn(H_2O)_6}]^{2+}$ has the maximum number of unpaired electrons.

**Statement II :** The number of pairs among ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CO)_4}]}$, ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CN)_4}]^{2-}}$ and ${[\mathrm{Ni(CO)_4}],[\mathrm{Ni(CN)_4}]^{2-}}$ that contain only diamagnetic species is two.

In the light of the above statements, choose the correct answer from the options given below :

Accepted Answer

B. 1. **Statement I**

Compare the number of unpaired electrons in each complex.

* $[\mathrm{Cu(NH_3)_4}]^{2+}$:
     $$
     \mathrm{Cu}^{2+};(3d^9)
     $$
     It has **1 unpaired electron**.

* $[\mathrm{Ni(en)_3}]^{2+}$:
     $$
     \mathrm{Ni}^{2+};(3d^8)
     $$
     Octahedral complex with **2 unpaired electrons**.

* $[\mathrm{Ni(NH_3)_6}]^{2+}$:
     $$
     \mathrm{Ni}^{2+};(3d^8)
     $$
     Octahedral complex with **2 unpaired electrons**.

* $[\mathrm{Mn(H_2O)_6}]^{2+}$:
     $$
     \mathrm{Mn}^{2+};(3d^5)
     $$
     Since $\mathrm{H_2O}$ is a weak-field ligand, the complex is high spin and has **5 unpaired electrons**.

Therefore, $[\mathrm{Mn(H_2O)_6}]^{2+}$ has the maximum number of unpaired electrons.

Hence, **Statement I is true**.

2. **Statement II**

Determine whether both species in each pair are diamagnetic.

* ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CO)_4}]}$

* $[\mathrm{NiCl_4}]^{2-}$ is **paramagnetic**.
     * $[\mathrm{Ni(CO)_4}]$ is **diamagnetic**.
     * **Not** a pair containing only diamagnetic species.

* ${[\mathrm{NiCl_4}]^{2-},[\mathrm{Ni(CN)_4}]^{2-}}$

* $[\mathrm{NiCl_4}]^{2-}$ is **paramagnetic**.
     * $[\mathrm{Ni(CN)_4}]^{2-}$ is **diamagnetic**.
     * **Not** a pair containing only diamagnetic species.

* ${[\mathrm{Ni(CO)_4}],[\mathrm{Ni(CN)_4}]^{2-}}$

* Both complexes are **diamagnetic**.
     * This is **one** pair containing only diamagnetic species.

Thus, the number of such pairs is **1**, not **2**.

Hence, **Statement II is false**.

3. Since **Statement I is true** and **Statement II is false**, the correct answer is **Option B**.