JEE Main Chemistry Question (2026) — Solution

Step-by-step solution

To determine the magnetic nature of the given complexes, we analyze the oxidation state, electronic configuration, and the nature of the ligands for each central metal atom/ion: 1. [ MnBr _4]^ 2- : Mn^ 2+ has a 3d^5 configuration. Br^- is a weak field ligand. The complex is tetrahedral with 5 unpaired electrons. (Paramagnetic) 2. [ NiCl _4]^ 2- : Ni^ 2+ has a 3d^8 configuration. Cl^- is a weak field ligand. The complex is tetrahedral with 2 unpaired electrons. (Paramagnetic) 3. [ Ni(CN) _4]^ 2- : Ni^ 2+ has a 3d^8 configuration. CN^- is a strong field ligand. The complex is square planar (dsp^2) with all electrons paired. (Diamagnetic) 4. [ Ni(CO) _4]: Ni^0 has a 3d^8 4s^2 configuration, which becomes 3d^ 10 in the presence of the strong field ligand CO. The complex is tetrahedral with all electrons paired. (Diamagnetic) 5. [ CoF _6]^ 3- : Co^ 3+ has a 3d^6 configuration. F^- is a weak field ligand. The complex is octahedral and high spin (t_ 2g ^4 e_g^2) with 4 unpaired electrons. (Paramagnetic) 6. [ Fe(CN) _6]^ 4- : Fe^ 2+ has a 3d^6 configuration. CN^- is a strong field ligand. The complex is octahedral and low spin (t_ 2g ^6 e_g^0) with all electrons paired. (Diamagnetic) 7. [ Mn(CN) _6]^ 3- : Mn^ 3+ has a 3d^4 configuration. CN^- is a strong field ligand. The complex is octahedral and low spin (t_ 2g ^4 e_g^0) with 2 unpaired electrons. (Paramagnetic) 8. [ Ti(CN) _6]^ 3- : Ti^ 3+ has a 3d^1 configuration. The complex is octahedral (t_ 2g ^1 e_g^0) with 1 unpaired electron. (Paramagnetic) 9. [ Cu(H _2 O) _6]^ 2+ : Cu^ 2+ has a 3d^9 configuration. The complex is octahedral (t_ 2g ^6 e_g^3) with 1 unpaired electron. (Paramagnetic) 10. [ Co(C _2 O _4 ) _3]^ 3- : Co^ 3+ has a 3d^6 configuration. Oxalate ( C _2 O _4^ 2- ) acts as a strong field ligand for Co^ 3+ . The complex is octahedral and low spin (t_ 2g ^6 e_g^0) with all electrons paired. (Diamagnetic) Therefore, the paramagnetic complexes are [ MnBr _4]^ 2- , [ NiCl _4]^ 2- , [ CoF _6]^ 3- , [ Mn(CN) _6]^ 3- , [ Ti(CN) _6]^ 3- , and [ Cu(H _2 O) _6]^ 2+ . The total number of paramagnetic complexes is 6. Answer: 6