Difference Between High Spin and Low Spin Complexes

The key difference between high spin and low spin complexes is that high spin complexes contain unpaired electrons, whereas low spin complexes tend to contain paired electrons.

The terms high spin and low spin are related to coordination complexes. These are called spin states of complexes. We can determine these states using crystal field theory and ligand field theory. Generally, octahedral complexes and tetrahedral complexes are high spin, while square planar complexes are low spin.

CONTENTS

1. Overview and Key Difference
2. What are High Spin Complexes 
3. What are Low Spin Complexes 
4. Side by Side Comparison – High Spin vs Low Spin Complexes in Tabular Form
5. Summary

What are High Spin Complexes?

High spin complexes are coordination complexes containing unpaired electrons at high energy levels. Since they contain unpaired electrons, these high spin complexes are paramagnetic complexes. This means these complexes can be attracted to an external magnetic field.

Figure 01: Energy Splitting Diagram for High Spin Complexes

A high spin energy splitting of a compound occurs when the energy required to pair two electrons is greater than the energy required to place an electron in a high energy state. Usually, octahedral and tetrahedral coordination complexes are high spin complexes.

What are Low Spin Complexes?

Low spin complexes are coordination complexes containing paired electrons at low energy levels. Since there are no unpaired electrons in the low spin complexes (all the electrons are paired), they are diamagnetic. This means these compounds cannot be attracted to an external magnetic field. The low energy splitting of a compound occurs when the energy required to pair two electrons is lower than the energy required to place an electron in a low energy state. Usually, square planar coordination complexes are low spin complexes.

Figure 02: Energy Splitting Diagram for Low Spin Complexes

In order to determine whether a given coordination complex is a high spin complex or a low spin complex, we can use the following tips.

1. Determination of the shape of the complex
2. Determination of the oxidation state of the metal centre
3. Determination of the d electron configuration of the metal
4. Determination of the crystal field diagram of the complex with regards to its geometry
5. Determination whether the splitting energy is greater than the pairing energy

What is the Difference Between High Spin and Low Spin Complexes?

The key difference between high spin and low spin complexes is that high spin complexes contain unpaired electrons, whereas low spin complexes tend to contain paired electrons. In high spin complexes, the energy required to pair up two electrons is greater than the energy required to place an electron of that complex in a high energy level. In contrast, in low spin complexes, the energy required to pair two electrons is lower than the energy required to place an electron in a high energy level.

Furthermore, another significant difference between high spin and low spin complexes is that the high spin complexes are paramagnetic because they have unpaired electrons, but low spin complexes are diamagnetic because they have all electrons paired. Generally, tetrahedral and octahedral compounds are high spin while square planar compounds are low spin.

Summary – High Spin vs Low Spin Complexes

There are two types of spin states of coordination complexes. These are the high spin state and the low spin state. The key difference between high spin and low spin complexes is that high spin complexes contain unpaired electrons, whereas low spin complexes tend to contain paired electrons.

Reference:

1. “High Spin and Low Spin Complexes.” Chemistry LibreTexts, Libretexts, 21 Nov. 2019, Available here.
2. “Spin States (d Electrons).” Wikipedia, Wikimedia Foundation, 18 Nov. 2019, Available here.
3. “High-Spin-and-Low-Spin-Complexes.” Chemistry Guru, Available here.

Image Courtesy:

1. “CFT-High Spin Splitting Diagram-Vector” By Offnfopt, reference image created by YanA – Own work created using File:CFT – High Spin Splitting Diagram 2.png as a reference (CC0) via Commons Wikimedia
2. “CFT-Low Spin Splitting Diagram-Vector” By Offnfopt, reference image created by YanA – Own work created using File:CFT – Low Spin Splitting Diagram 2.png as a reference (Public Domain) via Commons Wikimedia

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