Question

Draw a molecular orbital diagram for [Fe(NH3)6]2+, assuming sp3 hybridized lone-pair orbitals on NH3 (and neglecting the N-H orbitals). How would this diagram change if NH3 were substituted by Cl- to form [FeCl6]4- ? Based on this change, which compound would you expect to be more likely to occur in its low-spin configuration at an accessible temperature? Explain your answer, and include drawings as necessary.

Answer 1

The MO diagram for a system with has NH₃ as the ligand which has only sigma donor on them will look like the below diagram.

On the other hand a ligand like Cl^- which has both sigma as well as pi ligands which can act as donors.

So on this basis the NH₃ based system will form complex in low-spin configuration.