Spectrochemical Series

Spectrochemical Series

Spectrochemical Series

Spectrochemical Series

Spectrochemical series was founded in 1938 which is a list of ligands according to their strength. The nature of ligands play an important role to determine whether a transition metal complex is high-spin or low-spin. The splitting of d orbital energy is influenced by how strongly the ligand interacts with the metal. Ligands that interact only weakly(i.e. weaker crystal-field) produce little change in the d orbital energy levels, whereas ligands that interact strongly(i.e. stronger crystal-field) produce a larger change in d orbital energy levels. The spectrochemical series is a list of ligands arranged in increasing order based on the strength of their interaction with metal ions.
I < Br < S−2 < SCN < Cl < NO3 < N3 < F < OH < C2O4−2 < H2O < NCS < CH3CN < pyridine(py) < NH3 < ethylenediamine(en) < 2,2'-bipyridine(bipy) < 1,10-phenanthroline(phen) < NO2 < PPh3 < CN < CO.
The list can vary from one metal ion to another, since some ligands bind preferentially to certain metals. The order of field strength of the common ligands is independent of the nature of the metal cation and the geometry of the complex.

Why F is weak ligand and CO is a strong ligand?

Ligands which can form π bonds in addition to σ bond can cause greater splitting of metal d-orbitals than ligands which forms only σ bonds.
CO can form π bond as well as σ bond but F can form only σ bond, so, F is weak ligand and CO is a strong ligand.

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