: 250
The spectrochemical series ranks ligands by their crystal field splitting ability, determined experimentally from absorption spectra:
I- < Br- < S2- < Cl- < N3- < F- < OH- < ox2- < H2O < NCS- < py < NH3 < en < bipy < phen < NO2- < PPh3 < CN- < CO < NO+
Key trends: Halides are weak field (F- strongest among them). N-donor ligands are intermediate. Carbon-donor ligands (CN-, CO) are the strongest due to pi-backbonding — a phenomenon CFT cannot explain (requires MO theory).
The spin state of d4-d7 complexes depends on the competition between Delta (splitting energy) and P (pairing energy):
- Delta > P: Electrons pair in t2g → low spin (fewer unpaired electrons)
- Delta < P: Electrons occupy eg before pairing → high spin (maximum unpaired electrons)
For d1-d3 and d8-d10, only one configuration is possible regardless of field strength. Practical consequences: [Fe(H2O)6]2+ is high spin d6 (4 unpaired, paramagnetic) while [Fe(CN)6]4- is low spin d6 (0 unpaired, diamagnetic). Same metal ion, same oxidation state — the ligand determines everything.
Delta also increases with: (1) higher oxidation state of metal, (2) going down a group (3d < 4d < 5d), and (3) number of ligands (tetrahedral < octahedral).