Classification of Ligands by Field Strength: The Spectrochemical Series

Not every ligand affects the energies of a metal's d-orbitals to the same extent. Some ligands split the d-orbital energies only weakly; others split them very strongly. Arranging common ligands in order of the strength of this splitting gives an experimentally determined sequence called the spectrochemical series, and it is one of the most useful predictive tools in the whole of coordination chemistry — it is what ultimately decides whether a given complex is high-spin or low-spin (Section 5.14), and it strongly influences which shape a four-coordinate complex adopts (Section 5.7).

A simplified but reliable version of the series, running from the weakest-field ligands to the strongest, is:

Weak-field ligands sit at the left of the series — mainly the halide ions and other simple anions. They produce a comparatively small energy gap ($\Delta_o$) between the two split sets of d-orbitals in an octahedral complex, so small that it is almost always energetically cheaper for the metal's d-electrons to spread out singly across all five d-orbitals (obeying Hund's rule) than to pair up within the lower-energy set. Complexes with weak-field ligands are therefore typically high-spin (maximum number of unpaired electrons, for a given electron count).

Strong-field ligands sit at the right — most notably $\text{CN}^-$ and $\text{CO}$, and to a lesser extent $\text{NH}_3$ and $\text{en}$. They produce a large enough $\Delta_o$ that it becomes energetically cheaper for electrons to pair up within the lower-energy orbital set than to occupy the higher-energy set singly. Complexes with strong-field ligands of the same metal ion are therefore typically low-spin (minimum number of unpaired electrons possible for that electron count). …