JEE Main: Count Unpaired Electrons in These Complexes — Quick Trick!
❓ Question The number of paramagnetic metal complex species among the following — that have the same number of unpaired electrons — is asked for: [ Co(NH 3 ) 6 ] 3 + , [ Co(C 2 O 4 ) 3 ] 3 − , [ MnCl 6 ] 3 − , [ Mn(CN) 6 ] 3 − , [ CoF 6 ] 3 − , [ Fe(CN) 6 ] 3 − , [ FeF 6 ] 3 − We must find which of these are paramagnetic and then see how many of them share the same number of unpaired electrons. 🖼️ Question Image ✍️ Short Solution (stepwise) We analyse each complex: General steps: Determine metal oxidation state from complex charge. Write the d-electron count. Decide whether ligand is strong-field (low-spin) or weak-field (high-spin). Count unpaired electrons in octahedral splitting. 1. [ Co(NH 3 ) 6 ] 3 + [ \text{Co(NH}_3)_6 ]^{3+} Co oxidation: + 3 +3 → d 6 d^6 . NH₃ is a borderline/relatively strong ligand for Co³⁺ → typically low-spin . Low-spin d 6 d^6 d 6 → t 2 g 6 e g 0 t_{2g}^6 e_g^0 → 0 unpaired → di...