For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the ligand-field splitting parameter in ligand field theory, or the crystal-field splitting parameter in crystal field theory.
The spectrochemical series was first proposed in 1938 based on the results of absorption spectra of cobalt complexes.
However, it is known that "the spectrochemical series is essentially backwards from what it should be for a reasonable prediction based on the assumptions of crystal field theory.
"[3] This deviation from crystal field theory highlights the weakness of its assumption of purely ionic bonds between metal and ligand.
The order of the spectrochemical series can be derived from the understanding that ligands are frequently classified by their donor or acceptor abilities.