What are the differences between high-spin and low-spin complexes?

High-spin vs Low-spin Complexes

1. Electron arrangement
   • High-spin: Electrons spread out as much as possible, filling higher-energy orbitals before pairing.
   • Low-spin: Electrons pair up in lower-energy orbitals first, before going to higher ones.
2. Crystal field splitting (Δ)
   • High-spin: Happens when splitting is small (weak-field ligands like H₂O, Cl⁻).
   • Low-spin: Happens when splitting is large (strong-field ligands like CN⁻, CO).
3. Magnetic properties
   • High-spin: More unpaired electrons, so usually paramagnetic (attracted to magnets).
   • Low-spin: Fewer (or no) unpaired electrons, so often diamagnetic (weakly repelled by magnets).
4. Energy balance
   • High-spin: Electrons avoid pairing (saves pairing energy) but accept higher orbital energy.
   • Low-spin: Electrons use more pairing energy but stay in lower orbitals.

🔹 Example

• [Fe(H₂O)₆]²⁺ → High-spin (H₂O is weak field, gives 4 unpaired electrons).
• [Fe(CN)₆]⁴⁻ → Low-spin (CN⁻ is strong field, gives 0 unpaired electrons).

In short:

• High-spin = weak ligands, more unpaired electrons, paramagnetic.
• Low-spin = strong ligands, fewer unpaired electrons, diamagnetic.