MHT CET Chemistry Question (2026) — Solution

Step-by-step solution

In [ Co(NH _3)_6]^ 3+ , Co is in +3 oxidation state with 3d^6 configuration. Since NH_3 acts as a strong field ligand for Co^ 3+ , pairing of electrons takes place resulting in t_ 2g ^6 e_g^0 configuration. Number of unpaired electrons = 0. In [ CoF _6]^ 3- , Co is in +3 oxidation state with 3d^6 configuration. F^- is a weak field ligand, so no pairing occurs. The configuration is t_ 2g ^4 e_g^2. Number of unpaired electrons = 4. In [ NiCl _4]^ 2- , Ni is in +2 oxidation state with 3d^8 configuration. Cl^- is a weak field ligand, forming a tetrahedral complex with e^4 t_2^4 configuration. Number of unpaired electrons = 2. In [ Ni(CN) _4]^ 2- , Ni is in +2 oxidation state with 3d^8 configuration. CN^- is a strong field ligand, forming a square planar complex where all electrons are paired. Number of unpaired electrons = 0. Thus, [ CoF _6]^ 3- has the highest number of unpaired electrons. Answer: [ CoF _6]^ 3-