Among (i) [ Co ( NH ₃ )₆ ] Cl ₃ , (ii) [ Ni ( NH ₃ )₆ ] Cl ₂ , (iii) [ Cr ( H ₂ O )₆ ] Cl ₃ , (iv) [ Fe ( H ₂ O )₆ ] Cl ₂ the complex which is diamagnetic is

Step-by-step solution

(i) Co ⁺³=[ Ar ] 3 ~d ⁶ ~s ⁰ NH ₃ is a strong field ligand (ii) Ni ⁺²=[ Ar ] 3 ~d ⁸ 4 ~s ⁰ NH ₃ is a strong field ligand (iii) Cr ⁺³=[ Ar ] 3 ~d ³ 4 ~s ⁰ H ₂ O is a weak field ligand (iv) Fe ⁺²=[ Ar ] 3 ~d ⁶ 4 ~s ⁰ H ₂ O is a weak field ligand. So, [ Co ( NH ₃ )₆ ] Cl ₃ will be diamagnetic.