Ligands arranged by the crystal-field splitting Δ they produce — weak-field (small Δ) to strong-field (large Δ). This one ordering explains colour, magnetism and high- vs low-spin.
▪ Δ (splitting energy): rises left → right. Strong-field ligands (CN⁻, CO, NH₃, en) force electrons to pair → low-spin; weak-field ligands (I⁻, Br⁻, F⁻, H₂O) leave them unpaired → high-spin.
▪ Magnetism: more unpaired e⁻ → more paramagnetic. Low-spin complexes have fewer unpaired electrons (μ = √[n(n+2)] BM).
▪ Colour: larger Δ absorbs higher-energy (shorter-λ) light, so the complex shows the complementary colour. Strong-field ⇒ absorption shifts toward blue/violet.
▪ Pairing rule: if Δ > pairing energy P → low-spin; if Δ < P → high-spin. Δ is also larger for higher oxidation state and for 4d/5d metals than 3d.
| Field | Ligands | Result |
|---|---|---|
| Weak | I⁻, Br⁻, Cl⁻, F⁻, H₂O, OH⁻ | large complex, high-spin, more unpaired e⁻ |
| Medium | NH₃, en, EDTA, C₂O₄²⁻ | borderline; depends on metal & charge |
| Strong | NO₂⁻, CN⁻, CO | low-spin, electrons paired, small μ |
Tip: many teachers memorise the strong end as “…en, NH₃, NO₂⁻, CN⁻, CO” and the weak end as “I < Br < Cl < F < OH < H₂O”. See the CFT Splitting model in the lab to watch Δ change.