The spectrochemical series ranks ligands by their crystal field splitting ability: I- < Br- < S2- < Cl- < N3- < F- < OH- < ox2- < H2O < NCS- < CH3CN < py < NH3 < en < bipy < phen < NO2- < PPh3 < CN- < CO < NO+. Key observations: (1) Halides are weak field (F- is strongest among them due to small size). (2) N-donor ligands are generally stronger than O-donors. (3) Carbon-donor ligands (CN-, CO) are strongest due to pi-backbonding. (4) NH3 is stronger than H2O despite both being neutral — N is less electronegative, donates electron density more effectively.