Difference Between Spectrochemical Series and Nephelauxetic Series

The key difference between spectrochemical series and nephelauxetic series is that spectrochemical series of ligands have weak ligands in the left side and strong ligands in the right side whereas nephelauxetic series contains ligands with lesser ability to form covalent bonds with metal ions and ligands in the right side have a greater ability to form the covalent bonds.

The spectrochemical series and nephelauxetic series have ligands and metal ions arranged in an order. The parameter based on which the order of the ligands and metal ions is different from one series to the other.

1. Overview and Key Difference
2. What is a Spectrochemical Series
3. What is a Nephelauxetic Series
4. Side by Side Comparison – Spectrochemical Series vs Nephelauxetic Series in Tabular Form
5. Summary

What is a Spectrochemical Series?

Spectrochemical series is a list of ligands and metal ions arranged according to the ligand strength and the oxidation state of metal ions. This series is very useful in identifying whether a coordination complex is a high spin or low spin. The concept about spectrochemical series first evolved in 1938. This series was suggested based on the data obtained from absorption spectra of cobalt complexes.

Difference Between Spectrochemical Series and Nephelauxetic Series

Figure 01: A Part of the Spectrochemical Series

Ligands in this series are arranged according to the ligand strength. Here, ligands with a weak strength are placed in the left side of the spectrochemical series while strong ligands are placed in the right side of the series. The weak ligands are referred to as ligands that cannot cause forcible pairing of electrons in the 3d energy level, thus forming high spin complexes. Stronger ligands can cause forcible pairing of electrons in the 3d energy level and form low spin coordination complexes. The order of this series is arranged according to the donor or acceptor ability of the ligands.

In addition to ligands, metal ions can also be arranged in the spectrochemical series. The order of the ligand field split is used in arranging the metal ions. However, this order is independent of the identity of the ligand. Moreover, there are two observations regarding the spectrochemical series of metal ions; the value of the ligand field split increases with the increase of oxidation state of the metal ions. Also, the ligand field split value increases down a group of the periodic table.

What is Nephelauxetic Series?

A nephelauxetic series is a list of ligands or metal ions that are arranged according to their nephelauxetic effect. This term is mainly used for transition metal ions. The term nephelauxetic refers to a decrease in the Racah interelectronic repulsion parameter. The symbol of this parameter is “B”, and it is measured when a transition-metal free ion forms a complex with ligands.

A decrease in Racah parameter indicates less repulsion between two electrons in the d-orbitals of a metal, and the orbital is larger in the complex. This is called the electron cloud expansion of the complex, and it is important in determining the nephelauxetic effect.

When ligands are arranged in a list according to the measured nephelauxetic effect, it is closely similar to the spectrochemical series. However, this arrangement generally reflects the ability of the ligands to form covalent bonds with metal ions. The ligands in the left side have a lesser effect of forming a covalent bond with a metal ion while the ligands in the right side have a greater effect.

What is the Difference Between Spectrochemical Series and Nephelauxetic Series?

The key difference between spectrochemical series and nephelauxetic series is that spectrochemical series of ligands have weak ligands in the left side and strong ligands in the right side whereas nephelauxetic series contains ligands with small ability to form covalent bonds with metal ions and ligands in the right side have a large ability to form the covalent bonds.

Moreover, metal ions in a spectrochemical series are arranged in the order of ligand field split value (or the oxidation state) while the metal ions in a nephelauxetic series are arranged in the increasing order of nephelauxetic effect.

Below is a summary of the difference between spectrochemical series and nephelauxetic series in tabular form.

Summary – Spectrochemical Series vs Nephelauxetic Series

The spectrochemical series and nephelauxetic series have ligands and metal ions arranged in an order. The key difference between spectrochemical series and nephelauxetic series is that spectrochemical series of ligands have weak ligands on the left side and strong ligands on the right side whereas nephelauxetic series contains ligands with weaker ability to form covalent bonds with metal ions and ligands in the right side have a greater ability to form the covalent bonds.

Reference:

1. “B.2. Spectrochemical Series.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
2. “Spectrochemical Series.” Wikipedia, Wikimedia Foundation, 16 Apr. 2020, Available here.
3. “20.9A: The Nephelauxetic Effect.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.