Which of the following is paramagnetic?

Step-by-step solution

Fe ²⁺ electronic configuration is [ Ar ] 3 ~d ⁶ . Since CN is strong field ligand d electrons are paired . In Ni ( CO )₄ O . S . of Ni is zero electronic configuration is [ Ar 3 ~d ⁸ 4 ~s ² . In presence of CO it is [ Ar ] 3 ~d ¹⁰ 4 ~s ⁰, electrons are paired. Electronic configuration of Ni ²⁺[ Ar ] 3 ~d ⁸ 4 ~s ⁰, due to CN ⁻ ligand all electrons are paired. Co ³⁺ is [ Ar ] 3 ~d ⁶ since F is weak ligand hence paramagnetic.