The question requires determining which of the given complexes has zero Crystal Field Stabilization Energy (CFSE). Let's analyze each complex to identify the correct answer.
1. [Fe(H2O)6]3+
2. [Co(H2O)6]2+
3. [Co(H2O)6]3+
4. [Mn(H2O)6]3+
Based on the above analysis, the complex with zero CFSE is [Fe(H2O)6]3+, as all its d-electrons are unpaired and evenly distributed between t2g and eg orbitals, conferring no net crystal field stabilization.
The IUPAC name for the complex \( [\text{Co}(\text{ONO})(\text{NH}_3)_5]\text{Cl}_2 \) is