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.
Ligand Field Strength (Weak to Strong)
Compare Δoct with the pairing energy P: Δoct < P gives high spin, Δoct > P gives low spin.
| 1 | I- | iodide | weaker end |
| 2 | Br- | bromide | increasing Δoct → |
| 3 | Cl- | chloride | increasing Δoct → |
| 4 | F- | fluoride | increasing Δoct → |
| 5 | OH- | hydroxide | increasing Δoct → |
| 6 | C2O42- | oxalate (bidentate) | increasing Δoct → |
| 7 | H2O | aqua | increasing Δoct → |
| 8 | NH3 | ammine | increasing Δoct → |
| 9 | en | ethylenediamine | increasing Δoct → |
| 10 | NO2- | nitro (N-bound nitrite) | increasing Δoct → |
| 11 | 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 to d³ and d⁸ to 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.