The Spectrochemical Series

he value of the ligand field splitting parameter, ie. the amount by which the degeneracy of the d-orbitals is disturbed by the effect of the electrostatic field generated by the ligands, depends upon the identity of the ligands.

The ligand field splitting parameter can be measured by recording the optical absorption spectrum of the complex. The first absorption maximum in the spectrum is assigned to the t_2g-to-e_g transition, and the frequency of the absorption corresponds to the value of the splitting parameter.

If one investigates the series of compounds [CoX(NH₃)₅]ⁿ⁺, where X is I^–, Br^–, Cl^–, H₂O and NH₃, one finds that the colours range from purple, for I^–, through pink, for Cl^–, to yellow, for NH₃. This means that the frequency of the transition increases, and so the ligand field splitting parameter increases across the series.

The series obtained is independent of the identity of the metals at the center of the complex, and so the ligands can be arranged into the Spectrochemical Series:

I^– < Br^– < S^2- < SCN^– < Cl^– < NO₃^– < F^– < OH^– < C₂O₄^2- < H₂O < NCS^– < CH₃CN < NH₃ < en < bipy < phen < NO₂^– < PPh₃ < CN^– < CO

When a ligand causes there to be a large ligand field splitting parameter, it is said to generate a strong field, and when there is a small ligand field splitting parameter, it is said to be a weak field ligand. For example, CN^– is a strong field ligand and I^– is a weak field ligand.

For a given ligand, the value of Δ_o also depends systematically on the identity of the metal center. The metal ions, too, can be arranged in order of increasing Δ_o, and this order is largely independent of the identity of the ligand.

Mn²⁺ < Ni²⁺ < Co²⁺ < Fe²⁺ < V²⁺ < Fe³⁺ < Cr³⁺ < V³⁺ < Co³⁺

In general, it is not possible to say whether a given ligand will exert a strong field or a weak field on a given metal ion. However, when we consider the metal ion, two useful trends are observed:

Δ_o increases with increasing oxidation number, and

Δ_o increases down a group.