Spectrochemical Series

Trend Sequence:

$$\text{I}^- < \text{Br}^- < \text{SCN}^- < \text{Cl}^- < \text{S}^{2-} < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NCS}^- < \text{EDTA}^{4-} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$$

Concept: Ranks ligands according to their ability to split degenerate d-orbital energies ($$\Delta_o$$ or $$\Delta_t$$). Halides and oxygen-donor ligands form weak fields preserving high-spin states ($$\Delta_o < P$$), whereas nitrogen and carbon-donor ligands produce strong fields forcing electron pairing ($$\Delta_o > P$$).

Notation:

• $$\text{I}^-$$ = Weakest field ligand (minimum orbital splitting);
• $$\text{CO}$$ = Strongest field ligand (maximum orbital splitting);
• $$\Delta_o$$ = Crystal field splitting parameter in an octahedral field.