Spectrochemical Series Explained: Ligands, Color, and Metal Complexes in Chemistry

Spectrochemical Series: Comprehensive Overview

A spectrochemical series consists of a set of ligands arranged in ascending order of "strength" and a set of metal ions arranged in ascending order of element, oxidation group, and number. Based on how well ligands interact with metal ions, a method of classification known as the spectrochemical series has been developed. A key factor in deciding whether a transition metal complex has a high or low spin is the kind of ligands present. The d orbital energy splitting is influenced by the intensity of the ligand-metal interaction. Relatively moderate interactions between ligands change the d orbital energy levels just a little, whereas strong interactions between ligands significantly alter the d orbital energy levels. A shift in a band of the UV-Vis spectra happens when two similar complexes with two different ligands are compared, and this change gives birth to the spectrochemical series.

Spectrochemical Series: What is a Spectrochemical Series?

A spectrochemical series is a list of ligands arranged according to their "strength" and a list of metal ions arranged according to their element, oxidation number, and group. The ligands for a metal ion alter what is known as the crystal-field splitting parameter in crystal field theory or the ligand-field splitting parameter in ligand field theory as the energy difference between the d orbitals. The splitting parameter affects the ion's electronic and magnetic characteristics, such as its spin state, and optical properties, such as its color and absorption spectrum. On this list, the common ligands are listed in increasing order of ligand field strength-

According to "the spectrochemical series," it is basically the opposite of what it should be for a reasonable forecast based on crystal field theory's suppositions.

Spectrochemical Series: Who invented the Spectrochemical Series?

Shimura (1988) first proposed the Spectrochemical Series of the ligands based on experimental data and in good agreement with numerous qualitative studies that demonstrate the same order to explain the crystal field splitting between the eg (dz2, dx2-y2) and t2g (dxy, dxz, dyz) orbitals in transition metals. 

Spectrochemical Series:  History of Spectrochemical Series

In 1938, the spectrochemical series was created. This series is based on the findings of the cobalt complexes' absorption spectra. Essentially, this series is a collection of ligands that have been classified or categorized according to strength.  

Spectrochemical Series: Spectrochemical Series of Ligands

Below is a list of the ligands in the spectrochemical series, ordered from small to large-

Spectrochemical Series: Spectrochemical Series of Metals 

Additionally, various metal ions can be arranged in ascending order, which is as follows-

Spectrochemical Series:  Factors Affecting the Spectrochemical Series

The following are some of the factors affecting the spectrochemical series- 

1. Despite being experimentally determined, the series is challenging to theoretically explain.
2. The relative position, nevertheless, is determined by their growing contribution.
3. The strongest ligands in the series are those with a high negative charge, a small size, good donor and acceptor properties, and substantial crystal field splitting. In contrast, weak field ligands that function as -donors occupy the bottom position.

Read more about the Father of Chemistry and the Chemistry Periodic Table.

Spectrochemical Series: Significance of Spectrochemical Series

The following points highlight the importance of the electrochemical series-

1. The relative d-orbital splitting of two complexes containing the same metal ion with various ligands may be predicted using spectrochemical series. 
2. Similar to that, it is possible to anticipate the relative frequency of the absorption band.

Spectrochemical Series: Limitations of Spectrochemical Series

The spectrochemical series makes pure crystal field theory interpretation extremely challenging. Here is a list of some of the restrictions of spectrochemical series-

1. Although their dipole moments are in reverse order, NH3 splits more than H2O.
2. The effects would be greatest for anionic ligands, although they are at the low end.
3. The model also fails to explain why, for comparable complexes within a group, o only minimally fluctuates for some strong field ligands (like CN-).
4. The sequence is reversed even though neutral H2O should be above OH-.

Read more about the Electrochemical Series.

Spectrochemical Series: Uses of Spectrochemical Series

The following are some of the uses of spectrochemical series-

1. It is also possible to forecast the absorption band's relative frequency.
2. The relative d-orbital splitting of two complexes containing the same metal ion with various ligands may be predicted using this series.

Spectrochemical Series: Conclusion

A list of ligands arranged by strength, or a list of metal ions arranged by oxidation number, group, and element, makes up the spectrochemical series, which first appeared in 1938. In crystal field theory, ligands change the energy disparity between d orbitals, a phenomenon known as crystal field splitting. While ligands often give electrons away, metals tend to receive them. Weak field ligands and strong field ligands are the two types of ligands. On the other hand, weak field ligands are those that have little crystal field splitting and provide high values. Low spin values are the result of strong field ligands' significant crystal field splitting.