Spectrochemical Series
Ligands ranked by the crystal-field splitting Δo they produce, from weak field (small Δ) to strong field (large Δ). Choose a metal ion and slide along the series to see the d-electrons rearrange, the spin state flip and the colour shift.
Weak field to strong field
Tap a ligand to see why it sits where it does. It is also sent to the splitting lab below.
Build ML₆ and fill the d-orbitals
Strong-field ligands make Δo larger than the pairing energy P, so electrons pair up in t2g (low spin). Weak-field ligands leave Δo < P, so electrons spread out (high spin). Keys: ← → move along the series.
Diagram not to scale: the gap grows along the series and with metal charge. The dashed line marks where Δo equals P for this ion.
A complex shows the colour it does not absorb
A d–d transition absorbs light of energy Δo. The colour we see is the complementary colour, opposite on the wheel. As Δo grows, the absorption moves to shorter wavelength (towards blue and violet).
High spin or low spin?
Real octahedral complexes from the syllabus. Count d-electrons, place the ligand in the series, then decide.
Memory anchors and the reasons behind them
| Class | Ligands (NCERT order) | Typical result with 3d ions |
|---|---|---|
| Weak | I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O | Small Δo (Δo < P): high spin, more unpaired electrons. Exception: Co³⁺ is low spin even with H₂O and C₂O₄²⁻ (only F⁻ gives high-spin [CoF₆]³⁻). |
| Intermediate | NCS⁻ < edta⁴⁻ < NH₃ | Depends on the metal and its charge: [Co(NH₃)₆]³⁺ is low spin, but [Co(NH₃)₆]²⁺ and [Fe(NH₃)₆]²⁺ are high spin. |
| Strong | en < CN⁻ < CO (extended list adds bipy, phen, NO₂⁻ between en and CN⁻) | Large Δo (Δo > P): low spin for most d⁴–d⁷ ions, e.g. [Fe(CN)₆]⁴⁻ (diamagnetic) and [Fe(CN)₆]³⁻ (one unpaired e⁻). |