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A regium bond is a close-range interaction between a coinage-metal center—copper, silver, or gold—and an electron donor such as a Lewis base. In the σ-hole picture, an electron-poor region on the metal attracts the donor. The term grew out of computational work on metal nanoparticles, but it is also used to describe interactions in molecular structures. It does not mean that every contact involving copper, silver, or gold is a regium bond.
What makes an interaction a regium bond?
The name refers to the metal partner: the coinage metals copper (Cu), silver (Ag), and gold (Au). In the interaction, the metal acts as a Lewis-acid or electron-accepting partner, while a Lewis base supplies electron density. Later papers sometimes use coinage bond or coinage-metal bond for related interactions.
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One useful explanation is the σ-hole model. A metal’s electron distribution can be uneven; polarization may create a region of positive electrostatic potential that attracts an electron donor. The 2018 foundational study also discusses complementary regions of enhanced electron density, called σ-lumps. These concepts help describe the electrostatic pattern, but they should not be treated as a complete, universal explanation of every interaction labeled a regium bond.
How the concept began
Joakim Halldin Stenlid, Adam Johannes Johansson, and Tore Brinck introduced the term in a 2018 computational study of copper, silver, and gold nanoparticle clusters and their interactions with probe molecules. KTH’s profile of Brinck dates the introduction to 2017; that is distinct from the foundational paper’s 2018 publication date.
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The authors drew an analogy with hydrogen and halogen bonding: in each case, a region of positive electrostatic potential can attract an electron donor. Their nanoparticle work raised the possibility that a better understanding of these interactions could inform thinking about heterogeneous catalysis. That is a proposed relevance, not evidence that the paper established a particular catalyst improvement or commercial application.
How it compares with hydrogen and halogen bonding
| Feature | Regium bond | Hydrogen or halogen bond |
|---|---|---|
| Electrophilic partner | A coinage-metal center such as Cu, Ag, or Au | Hydrogen in a hydrogen bond; a halogen atom in a halogen bond |
| Common explanatory picture | A positive σ-hole on the metal may attract a Lewis base; some analyses also describe σ-lumps | A positive region associated with the donor atom may attract an electron donor |
| Settings in the cited examples | Nanoparticle surfaces and discrete molecular or crystal structures | Used here as an explanatory analogy; the cited sources do not establish a specific comparison system |
| Strength and bonding character | Depend on the specific system; some calculated cases have substantial covalent character | Not established as a single comparable value by the cited sources |
The analogy is useful for recognizing how an electron-poor region can attract a donor. It does not imply that these interaction types are physically identical in every respect.
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Where regium bonding appears beyond nanoparticles
A 2020 Chemical Communications paper reports the synthesis and X-ray characterization of a trichlorido(1-hexylcytosine)gold(III) complex and analyzes regium-bonding interactions in its structure. This example shows the terminology applied to a discrete molecular crystal, rather than only to a nanoparticle surface. It is a structural example, not proof of a general catalytic application.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsA 2023 paper uses “coinage bonds” and “regium bonds” as synonymous terms for close-shell interactions between an electron-deficient coinage atom and a nucleophilic Lewis-base center. Naming conventions can vary, so the surrounding description of the atoms and interaction matters.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why there is no universal regium-bond strength
Interaction energy and bonding character depend on the metal, its chemical environment, and the donor. A 2025 computational study by Zhou, Wang, Li, and Scheiner examined selected model complexes involving MR (M = Cu, Ag, or Au; R = Me, Cl, or CN) and methyl donors from BeMe₂, MgMe₂, or MeMgCl. For those modeled systems, the authors calculated interaction energies from −13 to −72 kcal/mol. The range belongs to those specific models; it is not a general strength scale for all regium bonds.
The study also describes substantial covalent contributions in its strongest gold-containing cases, alongside an approximately trigonal-bipyramidal, pentacoordinate carbon center. This illustrates why “regium bond” should not automatically be read as “purely noncovalent.” The reported values are computational results for selected complexes, not a universal experimentally measured quantity.
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What the term does—and does not—tell you
- It identifies an interaction category: a coinage-metal center interacting with an electron donor, in a specific molecular or surface setting.
- It points to a useful model: a σ-hole can help explain attraction between the metal and a Lewis base.
- It does not guarantee a single mechanism or strength: the degree of electrostatic and covalent character varies with the system.
- It does not by itself establish an application: proposed relevance to nanoparticle catalysis is motivation for study, not proof of a specific practical outcome.
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