Spectrochemical series

A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group and its identity. In crystal field theory, ligands modify the difference in energy between the d orbitals (Δ - ligand-field splitting parameter, or crystal-field splitting parameter), what is mainly reflected in differences in color of similar metal-ligand complexes.

A partial spectrochemical series listing of ligands from small Δ to large Δ:

I- < Br- < S2- < SCN- < Cl- < NO3- < N3- < F- < OH- < C2O42- ≈ H2O < NCS- < CH3CN < py (pyridine) <  NH3 < en (ethylenediamine) < bipy (2,2'-bipyridine) < phen (1,10-phenanthroline) < NO2- < PPh3 < CN- ≈ CO

(for a table, see the ligand page)