Spectrochemical Series
A ranking of ligands by the magnitude of the crystal field splitting they produce.
Formula / equation
I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < CN⁻ < CO
Unit
cm⁻¹
In depth
The order is largely independent of the metal, which makes it broadly predictive. It cannot be explained by electrostatics alone: ligands such as CO rank high because of pi back-bonding, so the series is really evidence for the covalent contribution that ligand field theory adds.
Examples in the real world
Replacing water with cyanide in an iron(II) complex changes it from paramagnetic to diamagnetic.