Spectrochemical series

The spectrochemical series is a term from ligand field theory . It arranges various ligands according to their ability to energetically split the d orbitals of a metal atom. The spectrochemical series of ligands was established in 1938 by the Japanese chemist R. Tsuchida.

Spectrochemical series of ligands

O ₂²⁻ <I ^- <Br ^- <S ²⁻ <SCN ^- <Cl ^- <N ₃^- <F ^- <NCO ^- <OH ^- <ONO ^- < ox ²⁻ <H ₂ O <NCS ^- <NC ^- < py <NH ₃ < en < bipy < phen <NO ₂^- <CNO ^- <CN ^- <CO

The ligands on the left in the row generate weak ligand field splitting and synchronization and can be viewed as strong Lewis bases or strong π donors. They tend to favor the formation of high-spin complexes.

The further to the right the ligands are, the greater the splitting of the ligand fields that is generated, which tends to lead to the formation of a low-spin complex.

Accordingly, the ligands on the right-hand side can be seen as strong Lewis acids or strong π-acceptors in relation to those on the left .

Criteria for the classification of the ligands in the series

Energy level diagram of the d orbitals of a metal ion in the octahedral ligand field

Electronegative ligands cause less splitting because they remove the electron density from the central particle and thus the interaction between the electrons is reduced. Larger ligands also cause less splitting of the ligand field, since their electrons are distributed over a larger volume.

From the molecular orbital theory , a connection with the ability of a ligand to build up a pi backbonding and the splitting of the d orbitals in the complex can be derived.

Spectrochemical series of metal ions

Mn ²⁺ <Ni ²⁺ <Co ²⁺ <Fe ²⁺ <V ²⁺ <Fe ³⁺ <Cr ³⁺ <V ³⁺ <Co ³⁺ <Mn ⁴⁺ <Mo ³⁺ <Rh ³⁺ <Pd ^{4 +} <Ir ³⁺ <Re ⁴⁺ <Pt ⁴⁺

The ligand field splitting also depends on the metal cation. The higher the charge of the metal cation, the higher the splitting and the more likely it is that low-spin complexes are formed. The metal ions on the left in this row cause a slight splitting of the ligand field and therefore tend to form high-spin complexes. The ones on the right cause a high degree of splitting and therefore tend to form low-spin complexes.

Criteria for the classification of the metals in the series

A higher oxidation state of the central particle has a direct influence on the ligands, which thereby come closer to the central particle. The electrostatic repulsion between the ligand electrons and the d electrons of the central particle is increased.

Coordination number = 4 tetrahedral complex

Coordination number = 4 square-planar complex

Example: nickel complexes

A criterion for the strength of the splitting is the geometry of four-coordinate complexes. Here, both tetrahedral and square-planar geometries are possible:

In the case of nickel (II) complexes, square-planar coordination polyhedra form with strongly splitting ligands and tetrahedral coordination polyhedra with less strongly splitting ligands.
If, on the other hand , one looks at nickel tetracarbonyl , one finds a tetrahedral geometry despite the strongly splitting carbonyl ligand: Nickel (0) contains 10 valence electrons in the valence state, which makes a square-planar geometry energetically unfavorable, since energetically higher-lying MOs are filled. A consideration with the ligand field theory is unnecessary here and the VSEPR model is used.

literature

AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .

References

↑ R. Tsuchida, in: Bull. Chem. Soc. Jpn. 1938 , 13 , 388 and 436.