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Biz, C.; Gracia, J.; Fianchini, M. Magnetism-Covalent Bonding Interplay. Encyclopedia. Available online: (accessed on 21 April 2024).
Biz C, Gracia J, Fianchini M. Magnetism-Covalent Bonding Interplay. Encyclopedia. Available at: Accessed April 21, 2024.
Biz, Chiara, Jose Gracia, Mauro Fianchini. "Magnetism-Covalent Bonding Interplay" Encyclopedia, (accessed April 21, 2024).
Biz, C., Gracia, J., & Fianchini, M. (2024, February 23). Magnetism-Covalent Bonding Interplay. In Encyclopedia.
Biz, Chiara, et al. "Magnetism-Covalent Bonding Interplay." Encyclopedia. Web. 23 February, 2024.
Magnetism-Covalent Bonding Interplay

Valence electrons are one of the main players in solid catalysts and in catalytic reactions, since they are involved in several correlated phenomena like chemical bonding, magnetism, chemisorption, and bond activation. This is particularly true in the case of solid catalysts containing d-transition metals, which exhibit a wide range of magnetic phenomena, from paramagnetism to collective behaviour. Indeed, the electrons of the outer d-shells are, on one hand, involved in the formation of bonds within the structure of a catalyst and on its surface, and, on the other, they are accountable for the magnetic properties of the material.

magnetism covalent bonding magnetic methods heterogeneous catalysis chemisorption processes

1. Introduction

Catalytic events occur through breaking and forming bonds over the surface of a catalyst in heterogeneous catalysis. The nature of such bonds is the result of several combined contributions (e.g., Coulomb, van der Waals, exchange, ionic …) which in turn depend on the electronic structure of the catalyst [1]. The majority of the catalysts of industrial interest are composed of elements with almost filled or fully filled 3d-, 4d-, or 5d-orbitals [1][2], which are also known to display magnetic properties [3][4]. Indeed, state-of-the-art catalysts such as platinum (Pt), iridium (Ir), palladium (Pd), iridium dioxide (IrO2), and ruthenium dioxide (RuO2) are classified as paramagnetic [5][6]. Paramagnetism occurs only in materials with unpaired electrons [3][7], and it is an expression of a deeper and multifaceted phenomenon known as magnetism (other expressions of this phenomenon are superparamagnetism, ferromagnetism, ferrimagnetism, and antiferromagnetism) [4]. Readers can find background information on magnetism and the magnetic properties of solid catalysts in the literature [8][9].
In the case of d-transition metal elements, their paramagnetic properties arise from the electrons of the outer d-shells [10][11]—the very same outer d-electrons that also participate in the formation of chemical bonds in materials and during catalytic reactions [11]. The relationship between magnetism and heterogeneous catalysis has been a debated topic in the scientific community since the middle of the 20th century. P.W. Selwood wrote in his work titled “Magnetism and catalysis” (1946) that the most active elements in catalysis, the d-transition metal elements, also exhibit intriguing magnetic properties, provided that not all chemical processes depend on magnetism [12]. A few years later, D.A. Dowden (1950) reported that a decrement in paramagnetism or ferromagnetism is observed for some metals during the chemisorption process. This decrement should be taken into account to provide a more complete description of this chemical event [1]. Later, R.J.H. Voorhoeve (1974) discussed whether magnetic ordering can play a role in the elementary steps of catalytic reactions in his work titled “Experimental Relationships between Catalysis and Magnetism[13]. As a final example, J.T. Richardson suggested in 1978 that maybe the property that makes solids paramagnetic (i.e., the presence of unpaired electrons) is the same property that also allows for the formation of bonds during chemisorption processes and the exchange of electrons during redox reactions [14]. This topic is still of modern interest since in the past few years, compositions containing magnetic 3d-transition metals as catalysts have been tested [15][16][17] and external magnetic fields (coupled or not with magnetic compositions) have been experimentally applied as a way to boost catalytic performances during catalytic reactions, particularly in oxygen catalysis [9][18][19].

2. Magnetism-Covalent Bonding Interplay in Structural Properties of Solids and during Chemisorption 

Bonds in solids are commonly classified as ionic, covalent, metallic, or molecular [20][21][22]. The valence electrons in ionic bonds undergo heteropolar binding which creates opposite charges that become subjected to an electrostatic interaction among themselves. In contrast, the valence electrons in covalent bonds undergo homopolar sharing among the involved atoms by forming electron pairs that are localized in the internuclear area. In metallic bonding, the valence electrons are delocalized among the atomic cores and defined as nondirectional bonds. Dispersive interactions (e.g., hydrogen bonds, van der Waals interactions, hydrophobic interactions, London dispersion forces, and dipole–dipole interactions) are responsible for the formation of these so-called molecular bonds by inducing dipole moments among neutral atoms in materials with closed-shell configurations (e.g., noble gasses at extremely low temperatures and crystals of organic molecules) [20][21]. These dispersive interactions are attractive forces whose existence does not depend on any charge density overlap between two (or more) atoms [20]. Nevertheless, the real description of a chemical bond in materials is, more often than not, the result of all these types of contributions, but when one contribution is predominant, the materials can be categorized into four different groups, as shown in Table 1.
Table 1. Examples of materials with predominant ionic, covalent, metallic, or molecular binding and their characteristics.
The strongest bonds encountered in materials are ionic and covalent bonds (see Table 1), which are also claimed to be the most common types of bonding involved in the control of the rate of the reaction during catalytic processes [1]. Nonetheless, one should not disregard the contributions of the other types of bonding in catalytic processes. To the researchers' knowledge, it is nearly impossible to separate bonding contributions in experiments. Nonetheless, the experimental quantification of the bond strength in the case of ionic crystals, for instance, can be carried out by descriptors like Madelung constants, which provide a measure of the electrostatic energy that binds the ions. For example, the Madelung constants of wurtzite (ZnS, M2+ X2− ion type, hexagonal), 1.64132, and of zinc oxide (ZnO, M2+ X2− ion type, hexagonal), 1.4985 [23], indicate that a higher amount of energy is required to break the crystal lattice of the former material because of the strong ionic contribution in its bonds. The separation and quantification of covalent, ionic, and non-covalent contributions in chemical bonds, in solid structures, and during catalytic events are more approachable via computational techniques and post-wavefunction analyses, like Atoms in Molecules (AIM) [24], NBO, RDG, DORI, and others.

3. Conclusions

The main requirements for a material to be employed as a solid catalyst in industrial catalytic processes are as follows: a high selectivity, a high stability under catalytic conditions, and a high activity. Identifying optimum catalytic compositions that possess all three desired requirements still represents a veritable challenge nowadays. Understanding the electronic, physical, chemical, and magnetic properties of solid catalysts themselves and during catalytic reactions is a starting point, but also a complex and multifaced end point in and of itself. Indeed, the relationship between magnetism and heterogeneous catalysis is still the object of a lot of debate. In this research, the researchers focused on the interplay between magnetism and covalent bonding. Experimental proofs of this interplay have been provided throughout the years, both in solids and during the chemisorption process. These experimental evidences prove that this interplay is accountable for chemical, physical, magnetic, and catalytic modifications of solid catalysts, and thus, it should not be disregarded when looking for optimum catalytic compositions. Moreover, even if this interplay represents only one aspect of the relationship between magnetism and heterogenous catalysis, it reveals the active role of magnetism in catalytic processes. To this end, future developments within this field should include the design of specific experiments targeting the relationship between magnetism and heterogeneous catalysis in chemisorption processes and in catalytic activation steps. Such experiments, in synergy with computational investigations, may help provide an improved and more complete understanding of this topic.


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