Spectrochemical Series and Spin States
A general-chemistry reference for comparing ligand field strength and deciding when octahedral complexes can be high spin or low spin.
| Order | Ligand | Name | Field-strength trend |
|---|---|---|---|
| 1 | I- | iodide | weaker end |
| 2 | Br- | bromide | increasing Δoct → |
| 3 | Cl- | chloride | increasing Δoct → |
| 4 | F- | fluoride | increasing Δoct → |
| 5 | H2O | aqua | increasing Δoct → |
| 6 | NH3 | ammine | increasing Δoct → |
| 7 | en | ethylenediamine | increasing Δoct → |
| 8 | NO2- | nitro (N-bound nitrite) | increasing Δoct → |
| 9 | CN- | cyanide | stronger end |
Important Notes
- The order runs from smaller to larger crystal-field splitting for otherwise comparable complexes. It is qualitative, not a universal numerical scale.
- For octahedral d⁴, d⁵, d⁶, and d⁷ complexes, compare Δoct with the electron-pairing energy P: Δoct < P gives high spin; Δoct > P gives low spin.
- Metal identity, oxidation state, 3d/4d/5d row, geometry, and the full ligand environment all affect Δ. A ligand name alone is not a complete spin-state calculation.
- For a mixed-ligand complex, there is no strongest-ligand-wins rule. Use a supplied Δoct/P comparison or experimental spin information. If neither is available and the d count is spin-sensitive, the spin state may be underdetermined.
- For octahedral d⁰–d³ and d⁸–d¹⁰ configurations, field strength does not create two alternative high-spin and low-spin fillings.
- Tetrahedral complexes are ordinarily treated as high spin in general chemistry because Δtet is small. Square-planar d⁸ complexes use a different splitting pattern and should be analyzed separately.
Source: OpenStax Chemistry 2e, Section 19.3 — Spectroscopic and Magnetic Properties of Coordination Compounds (CC BY 4.0)